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Investments (Amortized Cost And Fair Value Of Available-For-Sale and Held-To-Maturity Investment Portfolios) (Detail) - USD ($) $ in Millions | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Available For Sale Securities [Abstract] |  |  |
Total available-for-sale, amortized cost |  | $ 4,197 | $ 4,438
Gross unrealized gains |  | 85 | 91
Gross unrealized losses |  | (138) | (106)
Total available-for-sale, fair value |  | 4,144 | 4,423
Other-than-temporary impairments | [1] | (64) | (71)
Variable Interest Entity Primary Beneficiary [Member] |  |  |
Held To Maturity Securities [Abstract] |  |  |
Total held-to-maturity, amortized cost |  | 890 | 890
Gross unrealized gains |  | 11 | 26
Gross unrealized losses |  | 0 | 0
Total held-to-maturity investments, fair value |  | 901 | 916
Other-Than-Temporary Impairments | [1] | 0 | 0
Corporate Obligations [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |
Held To Maturity Securities [Abstract] |  |  |
Total held-to-maturity, amortized cost |  | 890 | 890
Gross unrealized gains |  | 11 | 26
Gross unrealized losses |  | 0 | 0
Total held-to-maturity investments, fair value |  | 901 | 916
Other-Than-Temporary Impairments | [1] | 0 | 0
Money Market Securities [Member] |  |  |
Available For Sale Securities [Abstract] |  |  |
Total available-for-sale, amortized cost |  | 0 | 179
Gross unrealized gains |  | 0 | 0
Gross unrealized losses |  | 0 | 0
Total available-for-sale, fair value |  | 0 | 179
Other-than-temporary impairments | [1] | 0 | 0
Perpetual Debt And Equity Securities [Member] |  |  |
Available For Sale Securities [Abstract] |  |  |
Total available-for-sale, amortized cost |  | 0 | 3
Gross unrealized gains |  | 0 | 1
Gross unrealized losses |  | 0 | 0
Total available-for-sale, fair value |  | 0 | 4
Other-than-temporary impairments | [1] | 0 | 0
Fixed Maturities [Member] |  |  |
Available For Sale Securities [Abstract] |  |  |
Total available-for-sale, amortized cost |  | 4,197 | 4,256
Gross unrealized gains |  | 85 | 90
Gross unrealized losses |  | (138) | (106)
Total available-for-sale, fair value |  | 4,144 | 4,240
Other-than-temporary impairments | [1] | (64) | (71)
Fixed Maturities [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |
Held To Maturity Securities [Abstract] |  |  |
Total held-to-maturity, amortized cost |  | 890 |
Total held-to-maturity investments, fair value |  | 901 |
Fixed Maturities [Member] | U S Treasury And Government [Member] |  |  |
Available For Sale Securities [Abstract] |  |  |
Total available-for-sale, amortized cost |  | 971 | 1,317
Gross unrealized gains |  | 27 | 34
Gross unrealized losses |  | (13) | (6)
Total available-for-sale, fair value |  | 985 | 1,345
Other-than-temporary impairments | [1] | 0 | 0
Fixed Maturities [Member] | US States And Political Subdivisions [Member] |  |  |
Available For Sale Securities [Abstract] |  |  |
Total available-for-sale, amortized cost |  | 808 | 840
Gross unrealized gains |  | 44 | 29
Gross unrealized losses |  | (9) | (12)
Total available-for-sale, fair value |  | 843 | 857
Other-than-temporary impairments | [1] | 9 | 0
Fixed Maturities [Member] | Foreign Governments [Member] |  |  |
Available For Sale Securities [Abstract] |  |  |
Total available-for-sale, amortized cost |  | 10 | 10
Gross unrealized gains |  | 0 | 0
Gross unrealized losses |  | 0 | 0
Total available-for-sale, fair value |  | 10 | 10
Other-than-temporary impairments | [1] | 0 | 0
Fixed Maturities [Member] | Corporate Obligations [Member] |  |  |
Available For Sale Securities [Abstract] |  |  |
Total available-for-sale, amortized cost |  | 1,677 | 1,332
Gross unrealized gains |  | 11 | 25
Gross unrealized losses |  | (104) | (80)
Total available-for-sale, fair value |  | 1,584 | 1,277
Other-than-temporary impairments | [1] | (74) | (72)
Fixed Maturities [Member] | Residential Mortgage-Backed Agency [Member] |  |  |
Available For Sale Securities [Abstract] |  |  |
Total available-for-sale, amortized cost |  | 306 | 365
Gross unrealized gains |  | 0 | 1
Gross unrealized losses |  | (7) | (4)
Total available-for-sale, fair value |  | 299 | 362
Other-than-temporary impairments | [1] | 0 | 0
Fixed Maturities [Member] | Residential Mortgage-Backed Non-Agency [Member] |  |  |
Available For Sale Securities [Abstract] |  |  |
Total available-for-sale, amortized cost |  | 32 | 35
Gross unrealized gains |  | 1 | 1
Gross unrealized losses |  | (3) | (4)
Total available-for-sale, fair value |  | 30 | 32
Other-than-temporary impairments | [1] | 0 | 0
Fixed Maturities [Member] | Commercial Mortgage-Backed [Member] |  |  |
Available For Sale Securities [Abstract] |  |  |
Total available-for-sale, amortized cost |  | 64 | 66
Gross unrealized gains |  | 0 | 0
Gross unrealized losses |  | (1) | 0
Total available-for-sale, fair value |  | 63 | 66
Other-than-temporary impairments | [1] | 0 | 0
Fixed Maturities [Member] | Collateralized Debt Obligations [Member] |  |  |
Available For Sale Securities [Abstract] |  |  |
Total available-for-sale, amortized cost |  | 113 | 116
Gross unrealized gains |  | 1 | 0
Gross unrealized losses |  | 0 | 0
Total available-for-sale, fair value |  | 114 | 116
Other-than-temporary impairments | [1] | 0 | 0
Fixed Maturities [Member] | Other Asset-Backed [Member] |  |  |
Available For Sale Securities [Abstract] |  |  |
Total available-for-sale, amortized cost |  | 216 | 175
Gross unrealized gains |  | 1 | 0
Gross unrealized losses |  | (1) | 0
Total available-for-sale, fair value |  | 216 | 175
Other-than-temporary impairments | [1] | $ 1 | $ 1
[1] | Represents unrealized gains or losses on other than temporarily impaired securities recognized in AOCI, which includes the non-credit component of impairments, as well as all subsequent changes in fair value of such impaired securities reported in AOCI.

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