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Investments (Gross Unrealized Losses Related To Available-For-Sale And Held-To-Maturity Investments) (Detail) - USD ($) $ in Millions | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | $ 1,365 |
Less than 12 months, unrealized losses | (13) |
12 months or longer, fair value | 438 |
12 months or longer, unrealized losses | (85) |
Total available-for-sale, fair value | 1,803 |
Total available-for-sale, unrealized losses | (98) |
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 | 1,365 | 1,872
Less than 12 months, unrealized losses | (13) | (62)
12 months or longer, fair value | 438 | 300
12 months or longer, unrealized losses | (85) | (86)
Total available-for-sale, fair value | 1,803 | 2,172
Total available-for-sale, unrealized losses | (98) | (148)
Fixed Maturities [Member] | U S Treasury And Government [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 279 | 432
Less than 12 months, unrealized losses | (2) | (10)
12 months or longer, fair value | 87 | 0
12 months or longer, unrealized losses | (3) | 0
Total available-for-sale, fair value | 366 | 432
Total available-for-sale, unrealized losses | (5) | (10)
Fixed Maturities [Member] | US States And Political Subdivisions [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 162 | 339
Less than 12 months, unrealized losses | (3) | (13)
12 months or longer, fair value | 53 | 18
12 months or longer, unrealized losses | (3) | (2)
Total available-for-sale, fair value | 215 | 357
Total available-for-sale, unrealized losses | (6) | (15)
Fixed Maturities [Member] | Foreign Government Debt [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 4 | 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 | 4 | 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 | 388 | 534
Less than 12 months, unrealized losses | (4) | (29)
12 months or longer, fair value | 135 | 52
12 months or longer, unrealized losses | (71) | (73)
Total available-for-sale, fair value | 523 | 586
Total available-for-sale, unrealized losses | (75) | (102)
Fixed Maturities [Member] | Residential Mortgage Backed Agency [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 344 | 436
Less than 12 months, unrealized losses | (4) | (9)
12 months or longer, fair value | 140 | 122
12 months or longer, unrealized losses | (4) | (3)
Total available-for-sale, fair value | 484 | 558
Total available-for-sale, unrealized losses | (8) | (12)
Fixed Maturities [Member] | Residential Mortgage Backed Non Agency [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 9 | 1
Less than 12 months, unrealized losses | 0 | 0
12 months or longer, fair value | 15 | 29
12 months or longer, unrealized losses | (4) | (6)
Total available-for-sale, fair value | 24 | 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 | 25 | 6
Less than 12 months, unrealized losses | 0 | 0
12 months or longer, fair value | 6 | 15
12 months or longer, unrealized losses | 0 | 0
Total available-for-sale, fair value | 31 | 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 | 5 | 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 | 5 | 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 | 149 | 112
Less than 12 months, unrealized losses | 0 | (1)
12 months or longer, fair value | 2 | 49
12 months or longer, unrealized losses | 0 | (2)
Total available-for-sale, fair value | 151 | 161
Total available-for-sale, unrealized losses | $ 0 | $ (3)

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