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Investments (Available-for-sale Investments) (Details Textual) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]          
Cash and cash equivalents $ 641   $ 641   $ 580
Total realized gains 1 $ 5 2 $ 22  
Total realized losses $ 3 $ 120 $ 8 $ 271