| **Reinsurance**                        | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   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| **Reinsurance Disclosures [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            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| Reinsurance                            | Note H. ReinsuranceThe Company cedes insurance to reinsurers to limit its maximum loss, provide greater diversification of risk, minimize exposures on larger risks and to exit certain lines of business.  The ceding of insurance does not discharge the primary liability of the Company.  A credit exposure exists with respect to property and casualty and life reinsurance ceded to the extent that any reinsurer is unable to meet its obligations or to the extent that the reinsurer disputes the liabilities assumed under reinsurance agreements.  Property and casualty reinsurance coverages are tailored to the specific risk characteristics of each product line and the Company's retained amount varies by type of coverage.  Reinsurance contracts are purchased to protect specific lines of business such as property and workers' compensation.  Corporate catastrophe reinsurance is also purchased for property and workers' compensation exposure.  Currently, most reinsurance contracts are purchased on an excess of loss basis.  The Company also utilizes facultative reinsurance in certain lines.  In addition, the Company assumes reinsurance, primarily through Hardy and as a member of various reinsurance pools and associations.The following table summarizes the amounts receivable from reinsurers at December 31, 2014 and 2013.Components of Reinsurance ReceivablesDecember 31   (In millions)2014 2013Reinsurance receivables related to insurance reserves:   Ceded claim and claim adjustment expenses$4,344 $4,972Ceded future policy benefits185 733Ceded policyholders' funds— 35Reinsurance receivables related to paid losses213 348Reinsurance receivables4,742 6,088Allowance for uncollectible reinsurance(48) (71)Reinsurance receivables, net of allowance for uncollectible reinsurance$4,694 $6,017The Company has established an allowance for uncollectible reinsurance receivables.  The Company reviews the allowance quarterly and adjusts the allowance as necessary to reflect changes in estimates of uncollectible balances.  The allowance may also be reduced related to write-offs of reinsurance receivable balances.The Company attempts to mitigate its credit risk related to reinsurance by entering into reinsurance arrangements with reinsurers that have credit ratings above certain levels, and by obtaining collateral.  On a limited basis, the Company may enter into reinsurance agreements with reinsurers that are not rated, primarily captive reinsurers.  The primary methods of obtaining collateral are through reinsurance trusts, letters of credit and funds withheld balances.  Such collateral was approximately $3.4 billion and $3.9 billion at December 31, 2014 and 2013.The Company's largest recoverables from a single reinsurer at December 31, 2014, including ceded unearned premium reserves were approximately $2,565 million from subsidiaries of Berkshire Hathaway Group, $244 million from subsidiaries of the Hartford Insurance Group and $185 million from subsidiaries of Wilton Re.  The recoverable from the Berkshire Hathaway Group includes amounts related to third-party reinsurance for which NICO has assumed the credit risk under the terms of the Loss Portfolio Transfer as discussed in Note F to the Consolidated Financial Statements.The effects of reinsurance on earned premiums and written premiums for the years ended December 31, 2014, 2013 and 2012 are shown in the following tables.Components of Earned Premiums(In millions)Direct Assumed Ceded Net Assumed/Net %2014 Earned Premiums         Property and casualty$9,452 $277 $3,073 $6,656 4.2%Accident and health508 48 — 556 8.6%Total earned premiums$9,960 $325 $3,073 $7,212 4.5%          2013 Earned Premiums         Property and casualty$9,063 $258 $2,609 $6,712 3.8%Accident and health511 48 — 559 8.6%Total earned premiums$9,574 $306 $2,609 $7,271 4.2%          2012 Earned Premiums         Property and casualty$8,354 $197 $2,229 $6,322 3.1%Accident and health512 47 — 559 8.4%Total earned premiums$8,866 $244 $2,229 $6,881 3.5%Components of Written Premiums(In millions)Direct Assumed Ceded Net Assumed/Net %2014 Written Premiums         Property and casualty$9,283 $276 $3,024 $6,535 4.2%Accident and health504 49 — 553 8.9%Total written premiums$9,787 $325 $3,024 $7,088 4.6%          2013 Written Premiums         Property and casualty$9,103 $249 $2,556 $6,796 3.7%Accident and health505 47 — 552 8.5%Total written premiums$9,608 $296 $2,556 $7,348 4.0%          2012 Written Premiums         Property and casualty$8,467 $169 $2,225 $6,411 2.6%Accident and health506 47 — 553 8.5%Total written premiums$8,973 $216 $2,225 $6,964 3.1%Included in the direct and ceded earned premiums for the years ended December 31, 2014, 2013 and 2012 are $2,643 million, $2,156 million and $1,794 million related to property business that is 100% reinsured under a significant third-party captive program.  The third-party captives that participate in this program are affiliated with the non-insurance company policyholders, therefore this program provides a means for the policyholders to self-insure this property risk.  The Company receives and retains a ceding commission.Accident and health premiums are from long duration contracts; property and casualty premiums are from short duration contracts.Insurance claims and policyholders' benefits reported on the Consolidated Statements of Operations are net of reinsurance recoveries of $1,379 million, $1,450 million and $1,443 million for the years ended December 31, 2014, 2013 and 2012, including $1,458 million, $712 million and $814 million related to the significant third-party captive program discussed above.  Reinsurance recoveries in 2014 were unfavorably affected by the commutation of a workers’ compensation reinsurance pool. |

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| - Definition
The entire disclosure pertaining to the existence, magnitude and information about insurance that has been ceded to or assumed from another insurance company, including the methodologies and assumptions used in determining recorded amounts.
+ References
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