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Available-for-sale investments (Details Textual) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]          
Cash and cash equivalents $ 545   $ 545   $ 458
Total realized gains 38 $ 215 113 $ 283  
Total realized losses $ 12 $ 192 $ 183 $ 313