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Investments - Available for sale fixed maturities and equity securities (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Schedule of Available-for-sale Securities [Line Items] |  |
Fixed maturities, Available for sale, Amortized Cost | $ 37,038 | $ 33,735
Available for sale (AFS) fixed maturities | 38,379 | 34,544
Equity securities, Available for sale, Amortized Cost | 1,321 | 1,351
Equity securities, Available for sale, Fair Value | 1,600 | 1,502
Total fixed maturities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fixed maturities, Available for sale, Amortized Cost | 37,038 | 33,735
Fixed maturities, Available for sale, Gross Unrealized, Gains | 1,423 | 1,054
Fixed maturities, Available for sale, Gross Unrealized, Losses | (82) | (245)
Fixed maturities, Available for sale, Net Unrealized, Gains (Losses) | 1,341 | 809
Available for sale (AFS) fixed maturities | 38,379 | 34,544
U.S. Government and government agencies |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fixed maturities, Available for sale, Amortized Cost | 244 | 315
Fixed maturities, Available for sale, Gross Unrealized, Gains | 1 | 3
Fixed maturities, Available for sale, Gross Unrealized, Losses | (3) | (3)
Fixed maturities, Available for sale, Net Unrealized, Gains (Losses) | (2) | 0
Available for sale (AFS) fixed maturities | 242 | 315
States, municipalities and political subdivisions |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fixed maturities, Available for sale, Amortized Cost | 6,887 | 6,650
Fixed maturities, Available for sale, Gross Unrealized, Gains | 254 | 200
Fixed maturities, Available for sale, Gross Unrealized, Losses | (18) | (69)
Fixed maturities, Available for sale, Net Unrealized, Gains (Losses) | 236 | 131
Available for sale (AFS) fixed maturities | 7,123 | 6,781
Foreign government |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fixed maturities, Available for sale, Amortized Cost | 124 | 131
Fixed maturities, Available for sale, Gross Unrealized, Gains | 3 | 5
Fixed maturities, Available for sale, Gross Unrealized, Losses | 0 | 0
Fixed maturities, Available for sale, Net Unrealized, Gains (Losses) | 3 | 5
Available for sale (AFS) fixed maturities | 127 | 136
Residential MBS |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fixed maturities, Available for sale, Amortized Cost | 2,884 | 3,367
Fixed maturities, Available for sale, Gross Unrealized, Gains | 349 | 281
Fixed maturities, Available for sale, Gross Unrealized, Losses | (6) | (13)
Fixed maturities, Available for sale, Net Unrealized, Gains (Losses) | 343 | 268
Available for sale (AFS) fixed maturities | 3,227 | 3,635
Commercial MBS |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fixed maturities, Available for sale, Amortized Cost | 927 | 1,446
Fixed maturities, Available for sale, Gross Unrealized, Gains | 36 | 49
Fixed maturities, Available for sale, Gross Unrealized, Losses | (1) | (2)
Fixed maturities, Available for sale, Net Unrealized, Gains (Losses) | 35 | 47
Available for sale (AFS) fixed maturities | 962 | 1,493
Asset-backed securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fixed maturities, Available for sale, Amortized Cost | 7,836 | 5,962
Fixed maturities, Available for sale, Gross Unrealized, Gains | 142 | 43
Fixed maturities, Available for sale, Gross Unrealized, Losses | (16) | (46)
Fixed maturities, Available for sale, Net Unrealized, Gains (Losses) | 126 | (3)
Available for sale (AFS) fixed maturities | 7,962 | 5,959
Corporate and other |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Fixed maturities, Available for sale, Amortized Cost | 18,136 | 15,864
Fixed maturities, Available for sale, Gross Unrealized, Gains | 638 | 473
Fixed maturities, Available for sale, Gross Unrealized, Losses | (38) | (112)
Fixed maturities, Available for sale, Net Unrealized, Gains (Losses) | 600 | 361
Available for sale (AFS) fixed maturities | 18,736 | 16,225
Total equity securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Equity securities, Available for sale, Amortized Cost | 1,321 | 1,351
Equity securities, Available for sale, Gross Unrealized, Gains | 302 | 181
Equity securities, Available for sale, Gross Unrealized, Losses | (23) | (30)
Equity securities, Available for sale, Net Unrealized, Gains (Losses) | 279 | 151
Equity securities, Available for sale, Fair Value | 1,600 | 1,502
Common stocks |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Equity securities, Available for sale, Amortized Cost | 786 | 879
Equity securities, Available for sale, Gross Unrealized, Gains | 263 | 160
Equity securities, Available for sale, Gross Unrealized, Losses | (22) | (23)
Equity securities, Available for sale, Net Unrealized, Gains (Losses) | 241 | 137
Equity securities, Available for sale, Fair Value | 1,027 | 1,016
Perpetual preferred stocks |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Equity securities, Available for sale, Amortized Cost | 535 | 472
Equity securities, Available for sale, Gross Unrealized, Gains | 39 | 21
Equity securities, Available for sale, Gross Unrealized, Losses | (1) | (7)
Equity securities, Available for sale, Net Unrealized, Gains (Losses) | 38 | 14
Equity securities, Available for sale, Fair Value | $ 573 | $ 486

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