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Investments (Available-for-sale, Textual) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]      
Cash and cash equivalents $ 580 $ 509  
Total realized gains 29 147 $ 304
Total realized losses $ 394 $ 213 $ 367