Available-for-sale investments (Details Textual) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Available-for-sale investments (Textual) [Abstract]
Cash	$ 454,000,000		$ 454,000,000		$ 370,000,000
Total realized gains	33,000,000	49,000,000	118,000,000	116,000,000
Total realized losses	26,000,000	11,000,000	44,000,000	30,000,000
Available for sale securities gross unrealized loss accumulated in investments	228,000,000		228,000,000		22,000,000
Continuous unrealized loss position, less than 12 months	$ 10,700,000,000		$ 10,700,000,000