Available-for-sale investments (Details Textual) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Available-for-sale investments (Textual) [Abstract]
Cash	$ 491		$ 491		$ 680
Total realized gains	31	32	147	169
Total realized losses	$ 11	$ 12	$ 41	$ 25