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Investments in Securities Other-than-temporary Impairments 1 (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]      
Other than Temporary Impairments, Amount $ 5 $ 35 $ 196
Available-for-sale securities [Member]      
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]      
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held $ 1,100 $ 1,900 $ 2,400