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Investments (Available-for-sale Investments) (Details Textual) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]      
Cash and cash equivalents $ 649   $ 580
Total realized gains 1 $ 17  
Total realized losses $ 5 $ 151