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Investments (Gross Unrealized Losses Related To Available-For-Sale And Held-To-Maturity Investments) (Detail) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | $ 1,204 |
Less than 12 months, unrealized losses | (13) |
12 months or longer, fair value | 617 |
12 months or longer, unrealized losses | (93) |
Total available-for-sale, fair value | 1,821 |
Total available-for-sale, unrealized losses | (106) |
Variable Interest Entity Primary Beneficiary [Member] |  |
Held To Maturity Securities [Abstract] |  |
Less than 12 months, fair value | 0 | $ 0
Less than 12 months, unrealized losses | 0 | 0
12 months or longer, fair value | 0 | 876
12 months or longer, unrealized losses | 0 | (14)
Total held-to-maturity, fair value | 0 | 876
Total held-to-maturity, unrealized losses | 0 | (14)
Corporate Obligations [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |
Held To Maturity Securities [Abstract] |  |
Less than 12 months, fair value | 0 | 0
Less than 12 months, unrealized losses | 0 | 0
12 months or longer, fair value | 0 | 876
12 months or longer, unrealized losses | 0 | (14)
Total held-to-maturity, fair value | 0 | 876
Total held-to-maturity, unrealized losses | 0 | (14)
Perpetual Debt And Equity Securities [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 0 |
Less than 12 months, unrealized losses | 0 |
12 months or longer, fair value | 0 |
12 months or longer, unrealized losses | 0 |
Total available-for-sale, fair value | 0 |
Total available-for-sale, unrealized losses | 0 |
Fixed Maturities [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 1,204 | 1,872
Less than 12 months, unrealized losses | (13) | (62)
12 months or longer, fair value | 617 | 300
12 months or longer, unrealized losses | (93) | (86)
Total available-for-sale, fair value | 1,821 | 2,172
Total available-for-sale, unrealized losses | (106) | (148)
Fixed Maturities [Member] | U S Treasury And Government [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 353 | 432
Less than 12 months, unrealized losses | (1) | (10)
12 months or longer, fair value | 124 | 0
12 months or longer, unrealized losses | (5) | 0
Total available-for-sale, fair value | 477 | 432
Total available-for-sale, unrealized losses | (6) | (10)
Fixed Maturities [Member] | US States And Political Subdivisions [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 203 | 339
Less than 12 months, unrealized losses | (8) | (13)
12 months or longer, fair value | 116 | 18
12 months or longer, unrealized losses | (4) | (2)
Total available-for-sale, fair value | 319 | 357
Total available-for-sale, unrealized losses | (12) | (15)
Fixed Maturities [Member] | Foreign Government Debt [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 8 | 5
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 | 8 | 5
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 | 425 | 534
Less than 12 months, unrealized losses | (3) | (29)
12 months or longer, fair value | 163 | 52
12 months or longer, unrealized losses | (77) | (73)
Total available-for-sale, fair value | 588 | 586
Total available-for-sale, unrealized losses | (80) | (102)
Fixed Maturities [Member] | Residential Mortgage Backed Agency [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 105 | 436
Less than 12 months, unrealized losses | (1) | (9)
12 months or longer, fair value | 156 | 122
12 months or longer, unrealized losses | (3) | (3)
Total available-for-sale, fair value | 261 | 558
Total available-for-sale, unrealized losses | (4) | (12)
Fixed Maturities [Member] | Residential Mortgage Backed Non Agency [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 0 | 1
Less than 12 months, unrealized losses | 0 | 0
12 months or longer, fair value | 14 | 29
12 months or longer, unrealized losses | (4) | (6)
Total available-for-sale, fair value | 14 | 30
Total available-for-sale, unrealized losses | (4) | (6)
Fixed Maturities [Member] | Commercial Mortgage Backed Securities [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 27 | 6
Less than 12 months, unrealized losses | 0 | 0
12 months or longer, fair value | 5 | 15
12 months or longer, unrealized losses | 0 | 0
Total available-for-sale, fair value | 32 | 21
Total available-for-sale, unrealized losses | 0 | 0
Fixed Maturities [Member] | Collateralized Debt Obligations [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 12 | 7
Less than 12 months, unrealized losses | 0 | 0
12 months or longer, fair value | 0 | 15
12 months or longer, unrealized losses | 0 | 0
Total available-for-sale, fair value | 12 | 22
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 | 71 | 112
Less than 12 months, unrealized losses | 0 | (1)
12 months or longer, fair value | 39 | 49
12 months or longer, unrealized losses | 0 | (2)
Total available-for-sale, fair value | 110 | 161
Total available-for-sale, unrealized losses | $ 0 | $ (3)

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