Available-for-sale investments (Details Textual) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Investments [Line Items]
Total realized gains	$ 18	$ 57	$ 54	$ 85
Total realized losses	27	$ 17	98	$ 43
Available-for-sale investments [Member]
Investments [Line Items]
Cash and cash equivalents	$ 479		$ 479		$ 438