| **Investments (Tables)**                                                                                                                                                                   | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
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| **Investments [Abstract]**                                                                                                                                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
| Net investment income                                                                                                                                                                      | Net Investment IncomePeriods ended September 30Three Months Nine Months(In millions)2013 2012 2013 2012Fixed maturity securities$504 $507 $1,501 $1,528Short term investments1 2 3 5Limited partnership investments93 89 303 184Equity securities3 4 9 10Mortgage loans6 5 17 13Trading portfolio (a)3 7 13 18Other1 — 2 3Gross investment income611 614 1,848 1,761Investment expense(14) (13) (40) (42)Net investment income$597 $601 $1,808 $1,719___________________(a)There were no net unrealized gains (losses) related to changes in fair value of trading securities still held included in net investment income for the three or nine months ended September 30, 2013 or 2012.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
| Net realized investment gains (losses)                                                                                                                                                     | Net Realized Investment Gains (Losses)Periods ended September 30Three Months Nine Months(In millions)2013 2012 2013 2012Net realized investment gains (losses):       Fixed maturity securities:       Gross realized gains$51 $75 $132 $193Gross realized losses(44) (49) (98) (120)Net realized investment gains (losses) on fixed maturity securities7 26 34 73Equity securities:       Gross realized gains3 5 10 10Gross realized losses(5) (20) (27) (24)Net realized investment gains (losses) on equity securities(2) (15) (17) (14)Derivatives(1) (1) (4) (1)Short term investments and other— (2) 5 8Net realized investment gains (losses)$4 $8 $18 $66                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
| Components of net other-than-temporary impairment losses recognized in earnings                                                                                                            | The components of net other-than-temporary impairment (OTTI) losses recognized in earnings by asset type are summarized in the following table.Periods ended September 30Three Months Nine Months(In millions)2013 2012 2013 2012Fixed maturity securities available-for-sale:       Corporate and other bonds$9 $7 $17 $23States, municipalities and political subdivisions— 17 — 17Asset-backed:       Residential mortgage-backed2 20 5 49Other asset-backed1 — 2 —Total asset-backed3 20 7 49U.S. Treasury and obligations of government-sponsored enterprises— — — 1Total fixed maturity securities available-for-sale12 44 24 90Equity securities available-for-sale:       Common stock3 1 5 5Preferred stock2 19 22 19Total equity securities available-for-sale5 20 27 24Net OTTI losses recognized in earnings$17 $64 $51 $114                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
| Summary of fixed maturity and equity securities                                                                                                                                            | Summary of Fixed Maturity and Equity SecuritiesSeptember 30, 2013Cost orAmortizedCost GrossUnrealizedGains GrossUnrealizedLosses EstimatedFairValue UnrealizedOTTILosses (Gains)(In millions)    Fixed maturity securities available-for-sale:         Corporate and other bonds$19,616 $1,695 $151 $21,160 $—States, municipalities and political subdivisions11,000 672 233 11,439 —Asset-backed:         Residential mortgage-backed4,779 141 101 4,819 (33)Commercial mortgage-backed1,942 99 24 2,017 (3)Other asset-backed942 16 2 956 —Total asset-backed7,663 256 127 7,792 (36)U.S. Treasury and obligations of government-sponsored enterprises166 8 1 173 —Foreign government547 17 3 561 —Redeemable preferred stock94 12 — 106 —Total fixed maturity securities available-for-sale39,086 2,660 515 41,231 $(36)Total fixed maturity securities trading43 — — 43  Equity securities available-for-sale:         Common stock41 12 — 53  Preferred stock148 1 3 146  Total equity securities available-for-sale189 13 3 199  Total$39,318 $2,673 $518 $41,473  December 31, 2012Cost orAmortizedCost GrossUnrealizedGains GrossUnrealizedLosses EstimatedFairValue UnrealizedOTTILosses (Gains)(In millions)    Fixed maturity securities available-for-sale:         Corporate and other bonds$19,530 $2,698 $21 $22,207 $—States, municipalities and political subdivisions9,372 1,455 44 10,783 —Asset-backed:         Residential mortgage-backed5,745 246 71 5,920 (28)Commercial mortgage-backed1,692 147 17 1,822 (3)Other asset-backed929 23 — 952 —Total asset-backed8,366 416 88 8,694 (31)U.S. Treasury and obligations of government-sponsored enterprises172 11 1 182 —Foreign government588 25 — 613 —Redeemable preferred stock113 13 1 125 —Total fixed maturity securities available-for-sale38,141 4,618 155 42,604 $(31)Total fixed maturity securities trading29 — — 29  Equity securities available-for-sale:         Common stock38 14 — 52  Preferred stock190 7 — 197  Total equity securities available-for-sale228 21 — 249  Total$38,398 $4,639 $155 $42,882   |
| Securities in a gross unrealized loss position                                                                                                                                             | Securities in a Gross Unrealized Loss Position Less than 12 Months 12 Months or Longer TotalSeptember 30, 2013EstimatedFair Value GrossUnrealizedLosses EstimatedFair Value GrossUnrealizedLosses EstimatedFair Value GrossUnrealizedLosses(In millions)     Fixed maturity securities available-for-sale:           Corporate and other bonds$3,521 $146 $45 $5 $3,566 $151States, municipalities and political subdivisions2,413 179 122 54 2,535 233Asset-backed:           Residential mortgage-backed1,289 40 287 61 1,576 101Commercial mortgage-backed449 20 79 4 528 24Other asset-backed207 2 — — 207 2Total asset-backed1,945 62 366 65 2,311 127U.S. Treasury and obligations of government-sponsored enterprises21 1 — — 21 1Foreign government76 3 — — 76 3Total fixed maturity securities available-for-sale7,976 391 533 124 8,509 515Equity securities available-for-sale:           Preferred stock86 3 — — 86 3Total$8,062 $394 $533 $124 $8,595 $518 Less than 12 Months 12 Months or Longer TotalDecember 31, 2012EstimatedFair Value GrossUnrealizedLosses EstimatedFair Value GrossUnrealizedLosses EstimatedFair Value GrossUnrealizedLosses(In millions)     Fixed maturity securities available-for-sale:           Corporate and other bonds$846 $13 $108 $8 $954 $21States, municipalities and political subdivisions254 5 165 39 419 44Asset-backed:           Residential mortgage-backed583 5 452 66 1,035 71Commercial mortgage-backed85 2 141 15 226 17Total asset-backed668 7 593 81 1,261 88U.S. Treasury and obligations of government-sponsored enterprises23 1 — — 23 1Redeemable preferred stock28 1 — — 28 1Total$1,819 $27 $866 $128 $2,685 $155                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
| 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 summarizes the activity for the three and nine months ended September 30, 2013 and 2012 related to the pretax credit loss component reflected in Retained earnings on fixed maturity securities still held at September 30, 2013 and 2012 for which a portion of an OTTI loss was recognized in Other comprehensive income (loss).Periods ended September 30Three Months Nine Months(In millions)2013 2012 2013 2012Beginning balance of credit losses on fixed maturity securities$89 $99 $95 $92Additional credit losses for securities for which an OTTI loss was previously recognized1 2 2 23Credit losses for securities for which an OTTI loss was not previously recognized— — — 2Reductions for securities sold during the period(7) (3) (14) (11)Reductions for securities the Company intends to sell or more likely than not will be required to sell— — — (8)Ending balance of credit losses on fixed maturity securities$83 $98 $83 $98                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
| Contractual maturity                                                                                                                                                                       | Contractual Maturity September 30, 2013 December 31, 2012(In millions)Cost orAmortizedCost EstimatedFairValue Cost orAmortizedCost EstimatedFairValueDue in one year or less$2,475 $2,516 $1,648 $1,665Due after one year through five years10,687 11,304 13,603 14,442Due after five years through ten years10,586 10,901 8,726 9,555Due after ten years15,338 16,510 14,164 16,942Total$39,086 $41,231 $38,141 $42,604                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |

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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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Tabular disclosure of maturities of an entity's investments as well as any other information pertinent to the investments.
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27340-111563Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number FAS115-1/124-1 -Paragraph 18B -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.
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Tabular disclosure of realized gains and losses on investments reported in the statement of income.
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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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