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Investments (Summary of fixed maturity and equity securities) (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2013 | Dec. 31, 2012
--- | --- | ---
Cost or Amortized Cost | $ 38,710 | $ 38,398
Gross Unrealized Gains | 4,448 | 4,639
Gross Unrealized Losses | 158 | 155
Estimated Fair Value | 43,000 | 42,882
Corporate and other bonds [Member] |  |
Cost or Amortized Cost | 19,747 | 19,530
Gross Unrealized Gains | 2,562 | 2,698
Gross Unrealized Losses | 18 | 21
Estimated Fair Value | 22,291 | 22,207
Unrealized OTTI Losses (Gains) |  |
States, municipalities and political subdivisions [Member] |  |
Cost or Amortized Cost | 9,599 | 9,372
Gross Unrealized Gains | 1,408 | 1,455
Gross Unrealized Losses | 59 | 44
Estimated Fair Value | 10,948 | 10,783
Unrealized OTTI Losses (Gains) |  |
Residential mortgage-backed [Member] |  |
Cost or Amortized Cost | 5,518 | 5,745
Gross Unrealized Gains | 235 | 246
Gross Unrealized Losses | 70 | 71
Estimated Fair Value | 5,683 | 5,920
Unrealized OTTI Losses (Gains) | (49) | (28)
Commercial mortgage-backed [Member] |  |
Cost or Amortized Cost | 1,853 | 1,692
Gross Unrealized Gains | 147 | 147
Gross Unrealized Losses | 10 | 17
Estimated Fair Value | 1,990 | 1,822
Unrealized OTTI Losses (Gains) | (3) | (3)
Other asset-backed [Member] |  |
Cost or Amortized Cost | 932 | 929
Gross Unrealized Gains | 23 | 23
Gross Unrealized Losses |  |
Estimated Fair Value | 955 | 952
Unrealized OTTI Losses (Gains) |  |
Total asset-backed [Member] |  |
Cost or Amortized Cost | 8,303 | 8,366
Gross Unrealized Gains | 405 | 416
Gross Unrealized Losses | 80 | 88
Estimated Fair Value | 8,628 | 8,694
Unrealized OTTI Losses (Gains) | (52) | (31)
U.S. Treasury and obligations of government-sponsored enterprises [Member] |  |
Cost or Amortized Cost | 170 | 172
Gross Unrealized Gains | 11 | 11
Gross Unrealized Losses |  | 1
Estimated Fair Value | 181 | 182
Unrealized OTTI Losses (Gains) |  |
Foreign government [Member] |  |
Cost or Amortized Cost | 528 | 588
Gross Unrealized Gains | 24 | 25
Gross Unrealized Losses |  |
Estimated Fair Value | 552 | 613
Unrealized OTTI Losses (Gains) |  |
Redeemable preferred stock [Member] |  |
Cost or Amortized Cost | 123 | 113
Gross Unrealized Gains | 14 | 13
Gross Unrealized Losses | 1 | 1
Estimated Fair Value | 136 | 125
Unrealized OTTI Losses (Gains) |  |
Total fixed maturity securities available-for-sale [Member] |  |
Cost or Amortized Cost | 38,470 | 38,141
Gross Unrealized Gains | 4,424 | 4,618
Gross Unrealized Losses | 158 | 155
Estimated Fair Value | 42,736 | 42,604
Unrealized OTTI Losses (Gains) | (52) | (31)
Total fixed maturity securities trading [Member] |  |
Cost or Amortized Cost, Trading Securities | 63 | 29
Gross Unrealized Gains, Trading Securities |  |
Gross Unrealized Losses, Trading Securities |  |
Estimated Fair Value, Trading Securities | 63 | 29
Common stock [Member] |  |
Cost or Amortized Cost | 38 | 38
Gross Unrealized Gains | 17 | 14
Gross Unrealized Losses |  |
Estimated Fair Value | 55 | 52
Preferred stock [Member] |  |
Cost or Amortized Cost | 139 | 190
Gross Unrealized Gains | 7 | 7
Gross Unrealized Losses |  |
Estimated Fair Value | 146 | 197
Total equity securities available-for-sale [Member] |  |
Cost or Amortized Cost | 177 | 228
Gross Unrealized Gains | 24 | 21
Gross Unrealized Losses |  |
Estimated Fair Value | $ 201 | $ 249

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