| **Claim and Claim Adjustment Expense Reserves (Tables)**                                                  | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
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| **Liability for Claims and Claims Adjustment Expense [Abstract]**                                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
| Net prior year development                                                                                | The following tables and discussion present net prior year development recorded for Specialty, Commercial, International and Corporate & Other Non-Core segments.Three months ended September 30, 2015         (In millions)Specialty  Commercial International Corporate & OtherNon-Core TotalPretax (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) $— $(180)Three months ended September 30, 2014         (In millions)Specialty  Commercial International Corporate & Other Non-Core TotalPretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development$(79) $71 $(17) $(1) $(26)Pretax (favorable) unfavorable premium development(4) — 7 — 3Total pretax (favorable) unfavorable net prior year development$(83) $71 $(10) $(1) $(23)Nine months ended September 30, 2015         (In millions)Specialty  Commercial International Corporate & Other Non-Core TotalPretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development$(141) $— $(46) $— $(187)Pretax (favorable) unfavorable premium development(10) (17) 16 — (11)Total pretax (favorable) unfavorable net prior year development$(151) $(17) $(30) $— $(198)Nine months ended September 30, 2014         (In millions)Specialty  Commercial International Corporate & Other Non-Core TotalPretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development$(123) $179 $(32) $(1) $23Pretax (favorable) unfavorable premium development(12) (24) 6 — (30)Total pretax (favorable) unfavorable net prior year development$(135) $155 $(26) $(1) $(7) |
| Net prior year claim and allocated claim adjustment expense reserve development for Specialty segment     | The following table presents further detail of the net prior year claim and allocated claim adjustment expense reserve development (development) recorded for the Specialty segment.Periods ended September 30Three Months Nine Months(In millions)2015 2014 2015 2014Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development:       Medical Professional Liability$(19) $16 $(11) $17Other Professional Liability and Management Liability(37) (9) (41) (59)Surety(70) (79) (69) (78)Warranty— — 1 —Other(4) (7) (21) (3)Total pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development$(130) $(79) $(141) $(123)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
| Net prior year claim and allocated claim adjustment expense reserve development for Commercial segment    | The following table presents further detail of the development recorded for the Commercial segment.  A significant amount of the unfavorable development for the nine months ended September 30, 2014 relates to business classes which the Company has exited, but also includes Small Business where the Company has taken underwriting actions in an effort to improve profitability.Periods ended September 30Three Months Nine Months(In millions)20152014 2015 2014Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development:       Commercial Auto$— $13 $7 $52General Liability3 44 8 76Workers' Compensation(1) 25 22 75Property and Other(13) (11) (37) (24)Total pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development$(11) $71 $— $179                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
| Net prior year claim and allocated claim adjustment expense reserve development for International segment | The following table presents further detail of the development recorded for the International segment.Periods ended September 30Three Months Nine Months(In millions)2015 2014 2015 2014Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development:       Medical Professional Liability$(8) $(3) $(8) $(2)Other Professional Liability(11) 1 (16) (14)Liability(5) (3) (12) (9)Property & Marine(5) (11) (19) (10)Other(5) (1) 9 (7)Commutations— — — 10Total pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development$(34) $(17) $(46) $(32)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
| Impact of loss portfolio transfer on consolidated statements of operations                                | The following table displays the impact of the Loss Portfolio Transfer on the Condensed Consolidated Statements of Operations.Periods ended September 30Three Months Nine Months(In millions)2015 2014 2015 2014Net A&EP adverse development before consideration of LPT$— $— $150 $—Provision for uncollectible third-party reinsurance on A&EP— — — —Additional amounts ceded under LPT— — 150 —Retroactive reinsurance benefit recognized(4) (4) (75) (9)Pretax impact of deferred retroactive reinsurance benefit$(4) $(4) $75 $(9)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |

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Impact of the loss portfolio transfer on the consolidated statements of operations
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Pretax Favorable Unfavorable Net Prior Year Claim And Allocated Claim Adjustment Expense Reserve Development [Table Text Block]
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Net prior year claim and allocated claim adjustment expense reserve development for International segment
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| - Definition
Net prior year claim and allocated claim adjustment expense reserve development for Commercial segment
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Net prior year claim and allocated claim adjustment expense reserve development for Specialty segment
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