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Investments (Summary of fixed maturity and equity securities) (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Cost or Amortized Cost | $ 38,874 | $ 38,467
Gross Unrealized Gains | 3,397 | 2,744
Gross Unrealized Losses | 89 | 196
Estimated Fair Value | 42,182 | 41,015
Corporate and other bonds |  |
Cost or Amortized Cost | 17,210 | 17,711
Gross Unrealized Gains | 1,625 | 1,323
Gross Unrealized Losses | 28 | 76
Estimated Fair Value | 18,807 | 18,958
Unrealized OTTI Losses (Gains) | 0 | (1)
States, municipalities and political subdivisions |  |
Cost or Amortized Cost | 12,478 | 12,060
Gross Unrealized Gains | 1,551 | 1,213
Gross Unrealized Losses | 2 | 33
Estimated Fair Value | 14,027 | 13,240
Unrealized OTTI Losses (Gains) | (11) | (16)
Residential mortgage-backed |  |
Cost or Amortized Cost | 5,043 | 5,004
Gross Unrealized Gains | 109 | 120
Gross Unrealized Losses | 32 | 51
Estimated Fair Value | 5,120 | 5,073
Unrealized OTTI Losses (Gains) | (27) | (28)
Commercial mortgage-backed |  |
Cost or Amortized Cost | 1,840 | 2,016
Gross Unrealized Gains | 46 | 48
Gross Unrealized Losses | 14 | 24
Estimated Fair Value | 1,872 | 2,040
Unrealized OTTI Losses (Gains) | 0 | 0
Other asset-backed |  |
Cost or Amortized Cost | 1,083 | 1,022
Gross Unrealized Gains | 16 | 8
Gross Unrealized Losses | 5 | 5
Estimated Fair Value | 1,094 | 1,025
Unrealized OTTI Losses (Gains) | 0 | 0
Total asset-backed |  |
Cost or Amortized Cost | 7,966 | 8,042
Gross Unrealized Gains | 171 | 176
Gross Unrealized Losses | 51 | 80
Estimated Fair Value | 8,086 | 8,138
Unrealized OTTI Losses (Gains) | (27) | (28)
U.S. Treasury and obligations of government-sponsored enterprises |  |
Cost or Amortized Cost | 111 | 83
Gross Unrealized Gains | 2 | 10
Gross Unrealized Losses | 4 | 0
Estimated Fair Value | 109 | 93
Unrealized OTTI Losses (Gains) | 0 | 0
Foreign government |  |
Cost or Amortized Cost | 437 | 435
Gross Unrealized Gains | 9 | 13
Gross Unrealized Losses | 2 | 3
Estimated Fair Value | 444 | 445
Unrealized OTTI Losses (Gains) | 0 | 0
Redeemable preferred stock |  |
Cost or Amortized Cost | 10 | 18
Gross Unrealized Gains | 1 | 1
Gross Unrealized Losses | 0 | 0
Estimated Fair Value | 11 | 19
Unrealized OTTI Losses (Gains) | 0 | 0
Fixed maturity securities |  |
Cost or Amortized Cost | 38,212 | 38,349
Gross Unrealized Gains | 3,359 | 2,736
Gross Unrealized Losses | 87 | 192
Estimated Fair Value | 41,484 | 40,893
Unrealized OTTI Losses (Gains) | (38) | (45)
Total fixed maturity securities trading |  |
Cost or amortized cost, trading securities | 3 | 12
Estimated fair value, trading securities | 3 | 12
Common Stock |  |
Cost or Amortized Cost | 21 | 13
Gross Unrealized Gains | 7 | 6
Gross Unrealized Losses | 1 | 0
Estimated Fair Value | 27 | 19
Preferred stock |  |
Cost or Amortized Cost | 638 | 93
Gross Unrealized Gains | 31 | 2
Gross Unrealized Losses | 1 | 4
Estimated Fair Value | 668 | 91
Equity securities |  |
Cost or Amortized Cost | 659 | 106
Gross Unrealized Gains | 38 | 8
Gross Unrealized Losses | 2 | 4
Estimated Fair Value | $ 695 | $ 110

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