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Investments (Net change in unrealized gains (losses) on investments) (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] |  |  |
Total net change in unrealized gains on investments | $ 758 | $ 224 | $ (1,119)
Fixed maturity securities |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Total net change in unrealized gains on investments | 728 | 225 | (1,114)
Equity securities |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Total net change in unrealized gains on investments | 32 | (2) | (6)
Other |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Total net change in unrealized gains on investments | $ (2) | $ 1 | $ 1

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