|**Claim and Claim Adjustment Expense Reserves**                                                                | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  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|**Liability for Claims and Claims Adjustment Expense [Abstract]**                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Claim and Claim Adjustment Expense Reserves                                                                    | NoteE. Claim and Claim Adjustment Expense ReservesThe Company's property and casualty insurance claim and claim adjustment expense reserves represent the estimated amounts necessary to resolve all outstanding claims, including incurred but not reported (IBNR) claims as of the reporting date.  The Company's reserve projections are based primarily on detailed analysis of the facts in each case, the Company's experience with similar cases and various historical development patterns.  Consideration is given to such historical patterns as field reserving trends and claim settlement practices, loss payments, pending levels of unpaid claims and product mix, as well as court decisions, economic conditions including inflation and public attitudes.  All of these factors can affect the estimation of claim and claim adjustment expense reserves.Establishing claim and claim adjustment expense reserves, including claim and claim adjustment expense reserves for catastrophic events that have occurred, is an estimation process.  Many factors can ultimately affect the final settlement of a claim and, therefore, the necessary reserve.  Changes in the law, results of litigation, medical costs, the cost of repair materials and labor rates can all affect ultimate claim costs.  In addition, time can be a critical part of reserving determinations since the longer the span between the incidence of a loss and the payment or settlement of the claim, the more variable the ultimate settlement amount can be.  Accordingly, short-tail claims, such as property damage claims, tend to be more reasonably estimable than long-tail claims, such as workers' compensation, general liability and professional liability claims.  Adjustments to prior year reserve estimates, if necessary, are reflected in the results of operations in the period that the need for such adjustments is determined.  There can be no assurance that the Company's ultimate cost for insurance losses will not exceed current estimates.Catastrophes are an inherent risk of the property and casualty insurance business and have contributed to material period-to-period fluctuations in the Company's results of operations and/or equity.  The Company reported catastrophe losses, net of reinsurance, of$16 millionand$137 millionfor thethree and nine months endedSeptember 30, 2016.  Catastrophe losses in 2016 resulted primarily from U.S. weather-related events and the Fort McMurray wildfires. The Company reported catastrophe losses, net of reinsurance, of$14 millionand$103 millionfor thethree and nine months endedSeptember 30, 2015.Net Prior Year DevelopmentThe following tables and discussion present the net prior year development.Three months ended September 30, 2016(In millions)SpecialtyCommercialInternationalCorporate & Other Non-CoreTotalPretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development$(112)$(5)$(15)$—$(132)Pretax (favorable) unfavorable premium development—(3)(2)—(5)Total pretax (favorable) unfavorable net prior year development$(112)$(8)$(17)$—$(137)Three months ended September 30, 2015(In millions)SpecialtyCommercialInternationalCorporate & Other Non-CoreTotalPretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development$(130)$(11)$(34)$—$(175)Pretax (favorable) unfavorable premium development(2)(5)2—(5)Total pretax (favorable) unfavorable net prior year development$(132)$(16)$(32)$—$(180)Nine months ended September 30, 2016(In millions)SpecialtyCommercialInternationalCorporate& OtherNon-CoreTotalPretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development$(211)$(37)$(34)$—$(282)Pretax (favorable) unfavorable premium development(18)(7)(2)—(27)Total pretax (favorable) unfavorable net prior year development$(229)$(44)$(36)$—$(309)Nine months ended September 30, 2015(In millions)SpecialtyCommercialInternationalCorporate& OtherNon-CoreTotalPretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development$(141)$—$(46)$—$(187)Pretax (favorable) unfavorable premium development(10)(17)16—(11)Total pretax (favorable) unfavorable net prior year development$(151)$(17)$(30)$—$(198)SpecialtyThe following table presents further detail of the net prior year claim and allocated claim adjustment expense reserve development (development) recorded for the Specialty segment.Periods ended September 30Three MonthsNine Months(In millions)2016201520162015Pretax (favorable) unfavorable development:Medical Professional Liability$13$(19)$(17)$(11)Other Professional Liability and Management Liability(48)(37)(98)(41)Surety(63)(70)(63)(69)Warranty2—71Other(16)(4)(40)(21)Total pretax (favorable) unfavorable development$(112)$(130)$(211)$(141)Three Months2016Unfavorable development for medical professional liability was largely due to higher than expected frequency in accident years 2014 and 2015 in aging services.  Increased claims on a specific hospital policy in accident years 2014 and 2015 was also an unfavorable contributor although more than offset by favorable development relative to expectations in accident years 2013 and prior.Favorable development in other professional liability and management liability was primarily related to lower than expected frequency of claims and favorable outcomes on specific claims for accident years 2010 through 2014.Favorable development in surety coverages was primarily due to lower than expected frequency of large losses in accident years 2014 and prior.Favorable development for other coverages was due to better than expected claim frequency in commercial lines coverages provided to Specialty customers in accident years 2010 through 2015.2015Favorable development in medical professional liability was related to lower than expected severity in accident years 2008 through 2013.Favorable development in other professional liability and management liability was related to better than expected large loss emergence in financial institutions in accident years 2012 and prior.  Additional favorable development related to lower than expected severity in accident years 2009 through 2013 for directors and officers liability.Favorable development for surety coverages was primarily due to lower than expected frequency of large losses in accident years 2013 and prior.Nine Months2016Favorable development for medical professional liability was primarily due to lower than expected severities for individual healthcare professionals, allied facilities and hospitals in accident years 2011 and prior.  This was partially offset by unfavorable development in accident years 2012 and 2013 related to higher than expected large loss emergence in hospitals and higher than expected frequency and severity in accident years 2014 and 2015 in our aging services business.Favorable development in other professional liability and management liability was primarily related to favorable settlements on closed claims in accident years 2011 through 2013 in professional services.  Additional favorable development related to lower than expected frequency of claims and favorable outcomes on specific claims in accident years 2010 through 2014 in professional services.  This was partially offset by unfavorable development related to a specific financial institutions claim in accident year 2014, higher severities in accident year 2015, and  deterioration on credit crises-related claims in accident year 2009.Favorable development in surety coverages was primarily due to lower than expected frequency of large losses in accident years 2014 and prior.Favorable development for other coverages provided to Specialty customers was due to better than expected claim frequency in property coverages in accident year 2015 and commercial lines coverages in accident years 2010 through 2015.2015Overall, favorable development for medical professional liability was related to lower than expected severity in accident years 2008 through 2013.  Unfavorable development was recorded related to increased claim frequency in the aging services business for accident years 2013 and 2014.Overall, favorable development in other professional liability and management liability related to better than expected large loss emergence in financial institutions in accident years 2012 and prior.  Additional favorable development related to lower than expected severity in accident years 2009 through 2013 for directors and officers liability and lower than expected severity in accident years 2010 and prior for professional services.  Unfavorable development was related to increased claim frequency on public company management liability in accident years 2012 through 2014.Favorable development for surety coverages was primarily due to lower than expected frequency of large losses in accident years 2013 and prior.Favorable development for other coverages was due to better than expected claim frequency in property coverages provided to Specialty customers in accident year 2014.CommercialThe following table presents further detail of the development recorded for the Commercial segment.Periods ended September 30Three MonthsNine Months(In millions)2016201520162015Pretax (favorable) unfavorable development:Commercial Auto$(12)$—$(47)$7General Liability143(38)8Workers' Compensation(6)(1)4822Property and Other(1)(13)—(37)Total pretax (favorable) unfavorable development$(5)$(11)$(37)$—Three Months2016Favorable development for commercial auto was primarily due to lower than expected severities in accident years 2012 through 2015.Unfavorable development for general liability was primarily due to an increase in reported claims prior to the closing of the three year window set forth by the Minnesota Child Victims Act in accident years 2006 and prior.Favorable development for workers' compensation was primarily driven by lower than expected frequencies in accident years 2009 through 2014, partially offset by the estimated impact of recent Florida court rulings in accident years 2008 through 2015.2015Favorable development for property and other was primarily due to better than expected loss emergence on catastrophe events in accident year 2014.Nine Months2016Favorable development for commercial auto was primarily due to favorable settlements on claims in accident years 2010 through 2014 and lower than expected severities in accident years 2012 through 2015.Favorable development for general liability was primarily due to better than expected claim settlements in accident years 2012 through 2014 and better than expected severity on umbrella claims in accident years 2010 through 2013.  This was partially offset by unfavorable development related to an increase in reported claims prior to the closing of the three year window set forth by the Minnesota Child Victims Act in accident years 2006 and prior.Unfavorable development for workers' compensation was primarily due to higher than expected severity for Defense Base Act contractors and the estimated impact of recent Florida court rulings in accident years 2008 through 2015.   This was partially offset by favorable development related to lower than expected frequencies related to accident years 2009 through 2014.Unfavorable development for property and other was primarily due to higher than expected severity from a 2015 catastrophe event.  This was offset by favorable development primarily due to better than expected loss frequency in accident years 2013 through 2015.2015Unfavorable development for workers’ compensation was primarily due to higher than expected severity related to Defense Base Act contractors in accident years 2008 through 2013.Favorable development for property and other was primarily due to better than expected loss emergence from 2012 and 2014 catastrophe events and better than expected frequency of large claims in accident year 2014.The nine months also included unfavorable loss development related to an extra contractual obligation loss and losses associated with premium development.InternationalThe following table presents further detail of the development recorded for the International segment.Periods ended September 30Three MonthsNine Months(In millions)2016201520162015Pretax (favorable) unfavorable development:Medical Professional Liability$(2)$(8)$(3)$(8)Other Professional Liability(1)(11)16(16)Liability(2)(5)(21)(12)Property & Marine(9)(5)(16)(19)Other(1)(5)(10)9Total pretax (favorable) unfavorable development$(15)$(34)$(34)$(46)Three Months2016Favorable development for other professional liability was primarily due to favorable settlements on claims in accident years 2013 and prior.  This was largely offset by higher than expected unfavorable large loss emergence in accident years 2014 and 2015.Favorable development for property and marine was primarily due to favorable emergence of expected losses on a specific claim relating to the December 2015 United Kingdom (UK) Floods.2015Favorable development in medical professional liability was due to better than expected loss emergence on accident years 2011 to 2013.Favorable development in other professional liability was due to better than expected large loss emergence in accident years 2011 and prior.Favorable development in liability was due to better than expected large loss emergence in accident years 2012 and prior.Favorable development in property and marine was due to better than expected individual large loss emergence and favorable settlements on large claims in accident years 2013 and 2014.Nine Months2016Unfavorable development for other professional liability was primarily due to higher than expected large loss emergence in accident years 2011 through 2015, partially offset by favorable settlements on claims in accident years 2013 and prior.Favorable development for liability was primarily due to better than expected severity in accident years 2013 and prior.Favorable development for property and marine was primarily due to favorable emergence of expected losses on a specific claim relating to the December 2015 UK Floods.Favorable development for other coverages was primarily due to better than expected severity in auto liability in accident years 2011 through 2015.2015Favorable development in medical professional liability was due to better than expected loss emergence on accident years 2011 to 2013.Favorable development in other professional liability was due to better than expected large loss emergence in accident years 2011 and prior.Favorable development in liability was due to better than expected large loss emergence in accident years 2012 and prior.Favorable development in property and marine was due to better than expected individual large loss emergence and favorable settlements on large claims in accident years 2013 and 2014.Unfavorable development in other is due to higher than expected large losses in financial institutions and political risk, primarily in accident year 2014.Asbestos and Environmental Pollution (A&EP) ReservesIn 2010, Continental Casualty Company (CCC) together with several of the Company’s insurance subsidiaries completed a transaction with National Indemnity Company (NICO), a subsidiary of Berkshire Hathaway Inc., under which substantially all of the Company’s legacy A&EP liabilities were ceded to NICO through a Loss Portfolio Transfer (LPT).  At the effective date of the transaction, the Company ceded approximately$1.6 billionof net A&EP claim and allocated claim adjustment expense reserves to NICO under a retroactive reinsurance agreement with an aggregate limit of$4 billion.  The$1.6 billionof claim and allocated claim adjustment expense reserves ceded to NICO was net of$1.2 billionof ceded claim and allocated claim adjustment expense reserves under existing third-party reinsurance contracts.  The NICO LPT aggregate reinsurance limit also covers credit risk on the existing third-party reinsurance related to these liabilities.  The Company paid NICO a reinsurance premium of$2 billionand transferred to NICO billed third-party reinsurance receivables related to A&EP claims with a net book value of$215 million, resulting in total consideration of$2.2 billion.Through December 31, 2013, the Company recognized$0.9 billionof additional amounts ceded under the LPT.  As a result, the cumulative amounts ceded under the LPT exceeded the$2.2 billionconsideration paid, resulting in the NICO LPT moving into a gain position, requiring deferred retroactive reinsurance accounting treatment.  This deferred gain is recognized in earnings in proportion to actual paid recoveries under the LPT.  Over the life of the contract, there is no economic impact as long as any additional losses incurred are within the limit of the LPT.  In a period in which a change in the estimate of ceded incurred losses is recognized, the change to the deferred gain is cumulatively recognized in earnings as if the revised estimate was available at the effective date of the LPT.The following table presents the impact of the Loss Portfolio Transfer on the Condensed Consolidated Statements of Operations.Periods ended September 30Three MonthsNine Months(In millions)2016201520162015Net A&EP adverse development before consideration of LPT$—$—$200$150Provision for uncollectible third-party reinsurance on A&EP————Additional amounts ceded under LPT——200150Retroactive reinsurance benefit recognized(12)(4)(94)(75)Pretax impact of unrecognized deferred retroactive reinsurance benefit$(12)$(4)$106$75The Company completed its reserve review of A&EP reserves in the first quarter of 2016.  Based upon the Company's review, net unfavorable development prior to cessions to the LPT of$200 millionwas recognized.  The unfavorable development was driven by an increase in anticipated future expenses associated with determination of coverage, higher anticipated payouts associated with a limited number of historical accounts having significant asbestos exposures and higher than expected severity on pollution claims. This unfavorable development was ceded to NICO under the LPT; however, the Company’s Net income was negatively affected due to the application of retroactive reinsurance accounting, as only a portion of the additional amounts ceded under the LPT were recognized in that quarter.  All amounts recognized related to the LPT are recorded within Insurance claims and policyholders’ benefits in the Condensed Consolidated Statement of Operations.As ofSeptember 30, 2016andDecember 31, 2015, the cumulative amounts ceded under the LPT were$2.8 billionand$2.6 billion.  The unrecognized deferred retroactive reinsurance benefit was$347 millionand$241 millionas ofSeptember 30, 2016andDecember 31, 2015.NICO established a collateral trust account as security for its obligations to the Company.  The fair value of the collateral trust account was$2.5 billionand$2.8 billionas ofSeptember 30, 2016andDecember 31, 2015.  In addition, Berkshire Hathaway Inc. guaranteed the payment obligations of NICO up to the full aggregate reinsurance limit as well as certain of NICO’s performance obligations under the trust agreement. NICO is responsible for claims handling and billing and collection from third-party reinsurers related to the Company’s A&EP claims.
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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Three months ended September 30, 2016                                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|(In millions)                                                                                                  | Specialty                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      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|Provision for uncollectible third-party reinsurance on A&EP                                                    | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Additional amounts ceded under LPT                                                                             | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Retroactive reinsurance benefit recognized                                                                     | (12                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    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