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Available-for-sale investments (Details Textual) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Investments [Line Items]          
Cash and cash equivalents $ 464   $ 464   $ 458
Total realized gains 40 $ 31 75 $ 68  
Total realized losses $ 87 $ 54 $ 171 $ 121