Available-for-sale investments (Details Textual) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Investments [Line Items]
Total realized gains	$ 31	$ 18	$ 68	$ 54
Total realized losses	54	$ 27	121	$ 98
Available-for-sale investments [Member]
Investments [Line Items]
Cash and cash equivalents	$ 467		$ 467		$ 406