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Investments (Gross Unrealized Losses Related To Available-For-Sale And Held-To-Maturity Investments) (Detail) - USD ($) $ in Millions | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | $ 2,108 |
Less than 12 months, unrealized losses | (33) |
12 months or longer, fair value | 580 |
12 months or longer, unrealized losses | (105) |
Total available-for-sale, fair value | 2,688 |
Total available-for-sale, unrealized losses | (138) |
Fixed Maturities [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 2,108 | $ 1,204
Less than 12 months, unrealized losses | (33) | (13)
12 months or longer, fair value | 580 | 617
12 months or longer, unrealized losses | (105) | (93)
Total available-for-sale, fair value | 2,688 | 1,821
Total available-for-sale, unrealized losses | (138) | (106)
Fixed Maturities [Member] | U S Treasury And Government [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 456 | 353
Less than 12 months, unrealized losses | (6) | (1)
12 months or longer, fair value | 165 | 124
12 months or longer, unrealized losses | (7) | (5)
Total available-for-sale, fair value | 621 | 477
Total available-for-sale, unrealized losses | (13) | (6)
Fixed Maturities [Member] | US States And Political Subdivisions [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 190 | 203
Less than 12 months, unrealized losses | (3) | (8)
12 months or longer, fair value | 99 | 116
12 months or longer, unrealized losses | (6) | (4)
Total available-for-sale, fair value | 289 | 319
Total available-for-sale, unrealized losses | (9) | (12)
Fixed Maturities [Member] | Foreign Government Debt [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 4 | 8
Less than 12 months, unrealized losses | 0 | 0
12 months or longer, fair value | 0 | 0
12 months or longer, unrealized losses | 0 | 0
Total available-for-sale, fair value | 4 | 8
Total available-for-sale, unrealized losses | 0 | 0
Fixed Maturities [Member] | Corporate Obligations [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 1,139 | 425
Less than 12 months, unrealized losses | (20) | (3)
12 months or longer, fair value | 156 | 163
12 months or longer, unrealized losses | (84) | (77)
Total available-for-sale, fair value | 1,295 | 588
Total available-for-sale, unrealized losses | (104) | (80)
Fixed Maturities [Member] | Residential Mortgage Backed Agency [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 144 | 105
Less than 12 months, unrealized losses | (2) | (1)
12 months or longer, fair value | 117 | 156
12 months or longer, unrealized losses | (5) | (3)
Total available-for-sale, fair value | 261 | 261
Total available-for-sale, unrealized losses | (7) | (4)
Fixed Maturities [Member] | Residential Mortgage Backed Non Agency [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 0 | 0
Less than 12 months, unrealized losses | 0 | 0
12 months or longer, fair value | 16 | 14
12 months or longer, unrealized losses | (3) | (4)
Total available-for-sale, fair value | 16 | 14
Total available-for-sale, unrealized losses | (3) | (4)
Fixed Maturities [Member] | Commercial Mortgage Backed Securities [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 30 | 27
Less than 12 months, unrealized losses | (1) | 0
12 months or longer, fair value | 4 | 5
12 months or longer, unrealized losses | 0 | 0
Total available-for-sale, fair value | 34 | 32
Total available-for-sale, unrealized losses | (1) | 0
Fixed Maturities [Member] | Collateralized Debt Obligations [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 9 | 12
Less than 12 months, unrealized losses | 0 | 0
12 months or longer, fair value | 0 | 0
12 months or longer, unrealized losses | 0 | 0
Total available-for-sale, fair value | 9 | 12
Total available-for-sale, unrealized losses | 0 | 0
Fixed Maturities [Member] | Other Asset Backed [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 136 | 71
Less than 12 months, unrealized losses | (1) | 0
12 months or longer, fair value | 23 | 39
12 months or longer, unrealized losses | 0 | 0
Total available-for-sale, fair value | 159 | 110
Total available-for-sale, unrealized losses | $ (1) | $ 0

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