Available-for-sale investments (Details Textual) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Investments [Line Items]
Total realized gains	$ 19	$ 17	$ 73	$ 102
Total realized losses	58	$ 28	156	$ 71
Available-for-sale investments [Member]
Investments [Line Items]
Cash and cash equivalents	$ 455		$ 455		$ 438