| **Claim and Claim Adjustment Expense Reserves**                   | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      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| **Liability for Claims and Claims Adjustment Expense [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       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| Claim and Claim Adjustment Expense Reserves                       | Note E. 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 claims 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 $36 million and $29 million for the three months ended March 31, 2016 and 2015.  Catastrophe losses in 2016 related primarily to U.S. weather-related events.Net Prior Year DevelopmentThe following tables and discussion present the net prior year development.Three months ended March 31, 2016          (In millions)Specialty  Commercial International Corporate & OtherNon-Core TotalPretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development$(34) $(14) $(4) $— $(52)Pretax (favorable) unfavorable premium development(11) (2) (1) — (14)Total pretax (favorable) unfavorable net prior year development$(45) $(16) $(5) $— $(66)Three months ended March 31, 2015         (In millions)Specialty  Commercial International Corporate & Other Non-Core TotalPretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development$2 $(5) $(4) $— $(7)Pretax (favorable) unfavorable premium development(6) (1) 16 — 9Total pretax (favorable) unfavorable net prior year development$(4) $(6) $12 $— $2SpecialtyThe following table presents further detail of the net prior year claim and allocated claim adjustment expense reserve development (development) recorded for the Specialty segment.Three months ended March 31   (In millions)2016 2015Pretax (favorable) unfavorable development:   Medical Professional Liability$(7) $14Other Professional Liability and Management Liability(9) (3)Surety— 1Warranty2 —Other(20) (10)Total pretax (favorable) unfavorable development$(34) $22016Favorable development for medical professional liability was due to lower than expected severity in accident years 2011 and prior.  This was partially offset by unfavorable development in accident years 2012 and 2013 recorded related to higher than expected large loss emergence and higher than expected severity in accident years 2014 and 2015.Favorable development in other professional liability and management liability was primarily related to favorable settlements on claims in accident years 2011 through 2013.  This was partially offset by unfavorable development recorded related to a specific financial institutions claim in accident year 2014 and continued deterioration on claims in accident year 2009 related to the credit crisis.Favorable development for other coverages was primarily due to better than expected claim frequency in property coverages provided to Specialty customers in accident year 2015. 2015Overall, unfavorable development for medical professional liability was primarily related to increased frequency in the Aging Services book for accident years 2013 and 2014, partially offset by better than expected severity in accident years 2010 and prior.Favorable development for other coverage was primarily due to better than expected frequency in property coverages provided to Specialty customers in accident year 2014.CommercialThe following table presents further detail of the development recorded for the Commercial segment.Three months ended March 31(In millions)20162015Pretax (favorable) unfavorable development:   Commercial Auto$(15) $—General Liability(15) 4Workers' Compensation4 (1)Property and Other12 (8)Total pretax (favorable) unfavorable development$(14) $(5)2016Favorable development for commercial auto was primarily due to favorable settlements on claims in accident years 2011 through 2014.  Favorable development for general liability was primarily due to favorable settlements on claims in accident years 2013 and 2014. Unfavorable development for property and other was primarily due to higher than expected severity from a 2015 catastrophe event. 2015Favorable development for property and other was due to lower than expected loss emergence from 2014 catastrophe events.InternationalThe following table presents further detail of the development recorded for the International segment.Three months ended March 31   (In millions)2016 2015Pretax (favorable) unfavorable development:   Medical Professional Liability$— $—Other Professional Liability(1) —Liability— (5)Property & Marine(4) (6)Other1 7Total pretax (favorable) unfavorable development$(4) $(4)Asbestos and Environmental Pollution Reserves (A&EP)In 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 the NICO Loss Portfolio Transfer (Loss Portfolio Transfer or LPT).  At the effective date of the transaction, the Company ceded approximately $1.6 billion of 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 billion of claim and allocated claim adjustment expense reserves ceded to NICO was net of $1.2 billion of 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 billion and 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 billion of additional amounts ceded under the LPT.  As a result, the cumulative amounts ceded under the LPT exceeded the $2.2 billion consideration 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.Three months ended March 31 (In millions)2016 2015Net A&EP adverse development before consideration of LPT$200 $—Provision for uncollectible third-party reinsurance on A&EP— —Additional amounts ceded under LPT200 —Retroactive reinsurance benefit recognized(73) (5)Pretax impact of unrecognized deferred retroactive reinsurance benefit$127 $(5)The Company expected to complete its reserve review of A&EP reserves in the second quarter of 2016; however, during the first quarter of 2016 the Company received communication from NICO that their reserve review indicated a substantial increase in reserves.  As a result, the Company accelerated and completed its reserve review during the first quarter.  Based upon the Company's accelerated review, net unfavorable development prior to cessions to the LPT of $200 million was recognized; $146 million related to asbestos claims and $54 million related to environmental pollution claims.  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 reported earnings were 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 the current quarter.  Retroactive reinsurance accounting resulted in the recognition of a reinsurance benefit of $73 million in the current quarter, while the remaining $127 million of incurred losses covered by the LPT will be recognized as a benefit through future earnings in proportion to the actual paid recoveries subject to the NICO LPT.  All amounts recognized related to the LPT are recorded within  Insurance claims and policyholders’ benefits in the Condensed Consolidated Statement of Operations. As of March 31, 2016 and December 31, 2015, the cumulative amounts ceded under the LPT were $2.8 billionand $2.6 billion.  Cumulative losses recognized in the Company's Condensed Consolidated Statement of Operations that are covered by the LPT were $368 million and $241 million as of March 31, 2016 and December 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 $2.7 billion and $2.8 billion as of March 31, 2016 and December 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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| - References
No definition available.
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| - Definition
The entire disclosure for liabilities related to future policy benefits and unpaid claims and claim adjustments.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6482852&loc=d3e14931-158439Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14754-158437Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.13(a)(1),(b)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437
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