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Investments (Credit Loss Rollforward) (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Investments, Debt and Equity Securities [Abstract]    
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Cash Flows $ (1) $ 0
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]    
Balance, beginning of period 138 187
Reduction:    
Sales (maturities, pay downs or prepayments) of securities previously impaired as credit loss OTTI (9) (17)
Balance, end of period $ 128 $ 170