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Available-for-sale investments (Details Textual) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Investments [Line Items]          
Total realized gains $ 215 $ 19 $ 283 $ 73  
Total realized losses 192 $ 58 313 $ 156  
Available-for-sale investments [Member]          
Investments [Line Items]          
Cash and cash equivalents $ 415   $ 415   $ 406