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Investments (Net realized investment gains (losses)) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items] |  |  |
Net realized investment gains (losses) | $ 93 | $ 50 | $ (67)
Fixed maturity securities |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Gross realized gains | 186 | 204 | 131
Gross realized losses | (64) | (138) | (197)
Net realized investment gains (losses) | 122 | 66 | (66)
Equity securities |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Gross realized gains | 1 | 5 | 2
Gross realized losses | (1) | (10) | (25)
Net realized investment gains (losses) |  | (5) | (23)
Derivatives |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Net realized investment gains (losses) | (4) | (2) | 10
Short term investments and other |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Net realized investment gains (losses) | $ (25) | $ (9) | $ 12

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