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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, 2017 | Dec. 31, 2016
--- | --- | ---
Available For Sale Securities [Abstract] |  |  |
Total available-for-sale, amortized cost |  | $ 5,206 | $ 5,450
Gross unrealized gains |  | 118 | 129
Gross unrealized losses |  | (139) | (148)
Total available-for-sale securities, fair value |  | 5,185 | 5,431
Other-than-temporary impairments | [1] | (64) | (75)
Variable Interest Entity Primary Beneficiary [Member] |  |  |
Held To Maturity Securities [Abstract] |  |  |
Total held-to-maturity, amortized cost |  | 890 | 890
Gross unrealized gains |  | 0 | 0
Gross unrealized losses |  | (11) | (14)
Total held-to-maturity investments, fair value |  | 879 | 876
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 |  | 0 | 0
Gross unrealized losses |  | (11) | (14)
Total held-to-maturity investments, fair value |  | 879 | 876
Other-Than-Temporary Impairments | [1] | 0 | 0
Money Market Securities [Member] |  |  |
Available For Sale Securities [Abstract] |  |  |
Total available-for-sale, amortized cost |  | 267 | 517
Gross unrealized gains |  | 0 | 0
Gross unrealized losses |  | 0 | 0
Total available-for-sale securities, fair value |  | 267 | 517
Other-than-temporary impairments | [1] | 0 | 0
Perpetual Debt And Equity Securities [Member] |  |  |
Available For Sale Securities [Abstract] |  |  |
Total available-for-sale, amortized cost |  | 3 | 4
Gross unrealized gains |  | 1 | 1
Gross unrealized losses |  | 0 | 0
Total available-for-sale securities, fair value |  | 4 | 5
Other-than-temporary impairments | [1] | 0 | 0
Fixed Maturities [Member] |  |  |
Available For Sale Securities [Abstract] |  |  |
Total available-for-sale, amortized cost |  | 4,936 | 4,929
Gross unrealized gains |  | 117 | 128
Gross unrealized losses |  | (139) | (148)
Total available-for-sale securities, fair value |  | 4,914 | 4,909
Other-than-temporary impairments | [1] | (64) | (75)
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 |  | 879 |
Fixed Maturities [Member] | U S Treasury And Government [Member] |  |  |
Available For Sale Securities [Abstract] |  |  |
Total available-for-sale, amortized cost |  | 865 | 909
Gross unrealized gains |  | 31 | 30
Gross unrealized losses |  | (8) | (10)
Total available-for-sale securities, fair value |  | 888 | 929
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 |  | 1,364 | 1,382
Gross unrealized gains |  | 60 | 72
Gross unrealized losses |  | (20) | (15)
Total available-for-sale securities, fair value |  | 1,404 | 1,439
Other-than-temporary impairments | [1] | 0 | 0
Fixed Maturities [Member] | Foreign Governments [Member] |  |  |
Available For Sale Securities [Abstract] |  |  |
Total available-for-sale, amortized cost |  | 10 | 8
Gross unrealized gains |  | 1 | 0
Gross unrealized losses |  | (1) | 0
Total available-for-sale securities, fair value |  | 10 | 8
Other-than-temporary impairments | [1] | 0 | 0
Fixed Maturities [Member] | Corporate Obligations [Member] |  |  |
Available For Sale Securities [Abstract] |  |  |
Total available-for-sale, amortized cost |  | 1,495 | 1,352
Gross unrealized gains |  | 20 | 20
Gross unrealized losses |  | (90) | (102)
Total available-for-sale securities, fair value |  | 1,425 | 1,270
Other-than-temporary impairments | [1] | (63) | (73)
Fixed Maturities [Member] | Residential Mortgage-Backed Agency [Member] |  |  |
Available For Sale Securities [Abstract] |  |  |
Total available-for-sale, amortized cost |  | 796 | 871
Gross unrealized gains |  | 2 | 3
Gross unrealized losses |  | (13) | (12)
Total available-for-sale securities, fair value |  | 785 | 862
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 |  | 59 | 50
Gross unrealized gains |  | 1 | 1
Gross unrealized losses |  | (6) | (6)
Total available-for-sale securities, fair value |  | 54 | 45
Other-than-temporary impairments | [1] | (2) | (3)
Fixed Maturities [Member] | Commercial Mortgage-Backed [Member] |  |  |
Available For Sale Securities [Abstract] |  |  |
Total available-for-sale, amortized cost |  | 38 | 41
Gross unrealized gains |  | 0 | 0
Gross unrealized losses |  | 0 | 0
Total available-for-sale securities, fair value |  | 38 | 41
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 |  | 52 | 22
Gross unrealized gains |  | 0 | 0
Gross unrealized losses |  | 0 | 0
Total available-for-sale securities, fair value |  | 52 | 22
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 |  | 257 | 294
Gross unrealized gains |  | 2 | 2
Gross unrealized losses |  | (1) | (3)
Total available-for-sale securities, fair value |  | 258 | 293
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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