Available-for-sale investments (Details Textual) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Cash and cash equivalents	$ 509	$ 458
Realized gains	172	306	$ 132
Realized losses	$ 213	$ 367	$ 208