| **Investments (Tables)**                                                                                                                                                                   | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
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| **Investments [Abstract]**                                                                                                                                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
| Net investment income                                                                                                                                                                      | The significant components of Net investment income are presented in the following table.Three months ended March 31   (In millions)2015 2014Fixed maturity securities$443 $452Short term investments2 1Limited partnership investments114 73Equity securities3 2Mortgage loans8 6Trading portfolio2 3Other— 2Gross investment income572 539Investment expense(14) (13)Net investment income$558 $526                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
| Net realized investment gains (losses)                                                                                                                                                     | Net realized investment gains (losses) are presented in the following table.Three months ended March 31   (In millions)2015 2014Net realized investment gains (losses):   Fixed maturity securities:   Gross realized gains$33 $53Gross realized losses(21) (15)Net realized investment gains (losses) on fixed maturity securities12 38Equity securities:   Gross realized gains1 5Gross realized losses(1) —Net realized investment gains (losses) on equity securities— 5Derivatives(1) —Short term investments and other(1) 3Net realized investment gains (losses)$10 $46                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
| Components of Net other-than-temporary impairment losses recognized in earnings by asset type                                                                                              | The components of Net other-than-temporary impairment (OTTI) losses recognized in earnings by asset type are presented in the following table.Three months ended March 31   (In millions)2015 2014Fixed maturity securities available-for-sale:   Corporate and other bonds$5 $1States, municipalities and political subdivisions5 —Asset-backed - residential mortgage-backed1 1Total fixed maturity securities available-for-sale11 2Equity securities available-for-sale:   Common stock1 —Net OTTI losses recognized in earnings$12 $2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
| Summary of fixed maturity and equity securities                                                                                                                                            | The following tables present a summary of fixed maturity and equity securities.March 31, 2015Cost orAmortizedCost GrossUnrealizedGains GrossUnrealizedLosses EstimatedFairValue UnrealizedOTTILosses (Gains)(In millions)    Fixed maturity securities available-for-sale:         Corporate and other bonds$16,721 $1,867 $43 $18,545 $—States, municipalities and political subdivisions11,407 1,536 9 12,934 —Asset-backed:         Residential mortgage-backed4,998 241 13 5,226 (51)Commercial mortgage-backed2,151 114 5 2,260 (3)Other asset-backed1,109 15 1 1,123 —Total asset-backed8,258 370 19 8,609 (54)U.S. Treasury and obligations of government-sponsored enterprises24 6 — 30 —Foreign government390 19 — 409 —Redeemable preferred stock39 3 — 42 —Total fixed maturity securities available-for-sale36,839 3,801 71 40,569 $(54)Total fixed maturity securities trading36   36  Equity securities available-for-sale:         Common stock41 9 — 50  Preferred stock172 7 4 175  Total equity securities available-for-sale213 16 4 225  Total$37,088 $3,817 $75 $40,830  December 31, 2014Cost orAmortizedCost GrossUnrealizedGains GrossUnrealizedLosses EstimatedFairValue UnrealizedOTTILosses (Gains)(In millions)    Fixed maturity securities available-for-sale:         Corporate and other bonds$17,210 $1,721 $61 $18,870 $—States, municipalities and political subdivisions11,285 1,463 8 12,740 —Asset-backed:         Residential mortgage-backed5,028 218 13 5,233 (53)Commercial mortgage-backed2,056 93 5 2,144 (2)Other asset-backed1,234 11 10 1,235 —Total asset-backed8,318 322 28 8,612 (55)U.S. Treasury and obligations of government-sponsored enterprises26 5 — 31 —Foreign government438 16 — 454 —Redeemable preferred stock39 3 — 42 —Total fixed maturity securities available-for-sale37,316 3,530 97 40,749 $(55)Total fixed maturity securities trading19   19  Equity securities available-for-sale:         Common stock38 9 — 47  Preferred stock172 5 2 175  Total equity securities available-for-sale210 14 2 222  Total$37,545 $3,544 $99 $40,990         |
| Securities in a gross unrealized loss position                                                                                                                                             | The following tables present the estimated fair value and gross unrealized losses of fixed maturity and equity securities in a gross unrealized loss position by the length of time in which the securities have continuously been in that position. Less than 12 Months 12 Months or Longer TotalMarch 31, 2015EstimatedFair Value GrossUnrealizedLosses EstimatedFair Value GrossUnrealizedLosses EstimatedFair Value GrossUnrealizedLosses(In millions)     Fixed maturity securities available-for-sale:           Corporate and other bonds$840 $31 $139 $12 $979 $43States, municipalities and political subdivisions479 6 100 3 579 9Asset-backed:           Residential mortgage-backed318 5 148 8 466 13Commercial mortgage-backed175 3 62 2 237 5Other asset-backed187 1 5 — 192 1Total asset-backed680 9 215 10 895 19U.S. Treasury and obligations of government-sponsored enterprises3 — — — 3 —Foreign government13 — 1 — 14 —Total fixed maturity securities available-for-sale2,015 46 455 25 2,470 71Equity securities available-for-sale:           Preferred stock15 4 — — 15 4Total$2,030 $50 $455 $25 $2,485 $75 Less than 12 Months 12 Months or Longer TotalDecember 31, 2014EstimatedFair Value GrossUnrealizedLosses EstimatedFair Value GrossUnrealizedLosses EstimatedFair Value GrossUnrealizedLosses(In millions)     Fixed maturity securities available-for-sale:           Corporate and other bonds$1,330 $46 $277 $15 $1,607 $61States, municipalities and political subdivisions335 5 127 3 462 8Asset-backed:           Residential mortgage-backed293 5 189 8 482 13Commercial mortgage-backed264 2 99 3 363 5Other asset-backed607 10 7 — 614 10Total asset-backed1,164 17 295 11 1,459 28U.S. Treasury and obligations of government-sponsored enterprises3 — 4 — 7 —   Foreign government3 — 3 — 6 —Redeemable preferred stock3 — — — 3 —Total fixed maturity securities available-for-sale2,838 68 706 29 3,544 97Equity securities available-for-sale:        Preferred stock17 2 1 — 18 2Total$2,855 $70 $707 $29 $3,562 $99 |
| Activity related to the pretax fixed maturity credit loss component reflected within retained earnings for securities still held for which a portion of an OTTI loss was recognized in OCI | The following table presents the activity related to the pretax credit loss component reflected in Retained earnings on fixed maturity securities still held as of March 31, 2015 and 2014 for which a portion of an OTTI loss was recognized in Other comprehensive income.Three months ended March 31   (In millions)2015 2014Beginning balance of credit losses on fixed maturity securities$62 $74Reductions for securities sold during the period(1) (2)Reductions for securities the Company intends to sell or more likely than not will be required to sell— (3)Ending balance of credit losses on fixed maturity securities$61 $69                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
| Contractual maturity                                                                                                                                                                       | The following table presents available-for-sale fixed maturity securities by contractual maturity. March 31, 2015 December 31, 2014(In millions)Cost orAmortizedCost EstimatedFairValue Cost orAmortizedCost EstimatedFairValueDue in one year or less$2,011 $2,043 $2,479 $2,511Due after one year through five years8,760 9,340 9,054 9,605Due after five years through ten years12,401 13,108 12,055 12,584Due after ten years13,667 16,078 13,728 16,049Total$36,839 $40,569 $37,316 $40,749                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
| Summary of aggregate contractual or notional amounts and gross estimated fair values related to derivative financial instruments                                                           | The following tables present the aggregate contractual or notional amounts and estimated fair values related to derivative financial instruments.March 31, 2015Contractual/NotionalAmount Estimated Fair Value(In millions) Asset LiabilityWithout hedge designation     Currency forwards$18 $2 $—Equity warrants5 — —Embedded derivative on funds withheld liability184 — 5Total  $2 $5December 31, 2014Contractual/NotionalAmount Estimated Fair Value(In millions) Asset LiabilityWithout hedge designation     Currency forwards$9 $— $—Equity warrants5 — —Embedded derivative on funds withheld liability184 — 3Total  $— $3                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |

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Tabular disclosure of investment income derived from investments in debt and equity securities consisting of interest income earned from investments in debt securities and on cash and cash equivalents, dividend income from investments in equity securities, and income or expense derived from the amortization of investment related discounts or premiums, respectively. This table includes realized gains (losses) on trading securities.
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| - Definition
Tabular disclosure of the amount of other than temporary impairment (OTTI) related to credit losses recognized in earnings including: (a) the beginning balance of the amount related to credit losses on debt securities held by the investor at the beginning of the period for which a portion of an OTTI was recognized in other comprehensive income; (b) additions for the amount related to the credit loss for which an OTTI was not previously recognized; (c) reductions for securities sold during the period (realized); (d) reductions for securities for which the amount previously recognized in other comprehensive income was recognized in earnings because the investor intends to sell the security or more likely than not will be required to sell the security before recovery of its amortized cost basis; (e) additional increases to the amount related to the credit loss for which an OTTI was previously recognized when the investor does not intend to sell the security and it is not more likely than not that the investor will be required to sell the security before recovery of its amortized cost basis; (f) reductions for increases in cash flows expected to be collected that are recognized over the remaining life of the security; and (g) the ending balance of the amount related to credit losses on debt securities held by the entity at the end of the period for which a portion of an OTTI was recognized in other comprehensive income.
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Tabular disclosure of the location and fair value amounts of derivative instruments (and nonderivative instruments that are designated and qualify as hedging instruments) reported in the statement of financial position.
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| - Definition
For all investments in an unrealized loss position, including those for which other-than-temporary impairments have not been recognized in earnings (including investments for which a portion of an other-than-temporary impairment has been recognized in other comprehensive income), a tabular disclosure of the aggregate related fair value of investments with unrealized losses and the aggregate amount of unrealized losses (that is, the amount by which amortized cost basis exceeds fair value).
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