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Investments (Credit Loss Rollforward) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Investments, Debt and Equity Securities [Abstract]        
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Cash Flows $ 0 $ 0 $ (1) $ 0
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]        
Balance, beginning of period 128 170 138 187
Reduction:        
Sales (maturities, pay downs or prepayments) of securities previously impaired as credit loss OTTI (17) (5) (26) (22)
Balance, end of period $ 111 $ 165 $ 111 $ 165