| **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 the net prior year development.Three months ended September 30, 2016         (In millions)Specialty Commercial International Corporate & Other Non-Core TotalPretax (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) $— $(137)Three months ended September 30, 2015         (In millions)Specialty Commercial International Corporate & Other Non-Core TotalPretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development$(130) $(11) $(34) $— $(175)Pretax (favorable) unfavorable premium development(2) (5) 2 — (5)Total pretax (favorable) unfavorable net prior year development$(132) $(16) $(32) $— $(180)Nine months ended September 30, 2016          (In millions)Specialty  Commercial International Corporate & OtherNon-Core TotalPretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development$(211) $(37) $(34) $— $(282)Pretax (favorable) unfavorable premium development(18) (7) (2) — (27)Total pretax (favorable) unfavorable net prior year development$(229) $(44) $(36) $— $(309)Nine months ended September 30, 2015         (In millions)Specialty  Commercial International Corporate & Other Non-Core TotalPretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development$(141) $— $(46) $— $(187)Pretax (favorable) unfavorable premium development(10) (17) 16 — (11)Total pretax (favorable) unfavorable net prior year development$(151) $(17) $(30) $— $(198) |
| 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)2016 2015 2016 2015Pretax (favorable) unfavorable development:       Medical Professional Liability$13 $(19) $(17) $(11)Other Professional Liability and Management Liability(48) (37) (98) (41)Surety(63) (70) (63) (69)Warranty2 — 7 1Other(16) (4) (40) (21)Total pretax (favorable) unfavorable development$(112) $(130) $(211) $(141)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
| 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.Periods ended September 30Three Months Nine Months(In millions)20162015 2016 2015Pretax (favorable) unfavorable development:       Commercial Auto$(12) $— $(47) $7General Liability14 3 (38) 8Workers' Compensation(6) (1) 48 22Property and Other(1) (13) — (37)Total pretax (favorable) unfavorable development$(5) $(11) $(37) $—                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
| 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)2016 2015 2016 2015Pretax (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)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
| Impact Of Loss Portfolio Transfer On Consolidated Statements Of Operations [Table Text Block]             | The following table presents the impact of the Loss Portfolio Transfer on the Condensed Consolidated Statements of Operations.Periods ended September 30Three Months Nine Months(In millions)2016 2015 2016 2015Net A&EP adverse development before consideration of LPT$— $— $200 $150Provision for uncollectible third-party reinsurance on A&EP— — — —Additional amounts ceded under LPT— — 200 150Retroactive reinsurance benefit recognized(12) (4) (94) (75)Pretax impact of unrecognized deferred retroactive reinsurance benefit$(12) $(4) $106 $75                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |

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| - Definition
Impact of the loss portfolio transfer on the consolidated statements of operations
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| X                                                                                                                                                                                                                                                                                                                                                                                                                |
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| - Definition
Pretax Favorable Unfavorable Net Prior Year Claim And Allocated Claim Adjustment Expense Reserve Development [Table Text Block]
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| X                                                                                                                                                                                                                                                                                                                                                                                                       |
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| - Definition
Net prior year claim and allocated claim adjustment expense reserve development for International segment
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No definition available.
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| - Definition
Net prior year claim and allocated claim adjustment expense reserve development for Commercial segment
+ References
No definition available.
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| - Definition
Net prior year claim and allocated claim adjustment expense reserve development for Specialty segment
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No definition available.
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| - References
No definition available.
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