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Investments (Narrative) (Detail) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2017 USD ($) Security | Sep. 30, 2016 USD ($) | Sep. 30, 2017 USD ($) Security | Sep. 30, 2016 USD ($) | Dec. 31, 2016 USD ($)
Investment [Line Items] |  |  |  |  |
Net unrealized appreciation (depreciation) included in OCI | $ 1 | $ 13 | $ (2) | $ 57 |
Unrealized appreciation(depreciation) portion of AOCI with OTTI included in AOCI | $ 7 |  | $ 7 |  | $ 10
Percentage of mortgage-backed securities represented by investments in US government agency bonds | 82.00% |  | 82.00% |  | 81.00%
Total number of fixed maturities | Security | 30,933 |  | 30,933 |  |
Number of equity securities in an unrealized loss position | Security | 59 |  | 59 |  |
Total number of equity securities | Security | 317 |  | 317 |  |
Largest single unrealized loss in the equity securities | $ (1) |  | $ (1) |  |
Restricted assets in fixed maturities and short-term investments | 22,800 |  | 22,800 |  | $ 20,100
Restricted assets in cash | $ 105 |  | $ 105 |  | $ 103
Moodys Historical Mean Recovery Rate |  |  | 42.00% |  |
Company Assumed Recovery Rate |  |  | 32.00% |  |
Number of fixed maturities in an unrealized loss position | Security | 7,455 |  | 7,455 |  |
Largest single unrealized loss in the fixed maturities | $ 7 |  | $ 7 |  |
Corporate [Member] |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Credit losses recognized in net income | 3 | 4 | 5 | 28 |
Collateralized Mortgage Backed Securities [Member] |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Credit losses recognized in net income | $ 0 | $ 1 | $ 0 | $ 1 |

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