Available-for-sale investments (Details Textual) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 ATL Holdings [Member]	Sep. 30, 2013 Reclassified to Restricted Investments [Member]
Schedule of Equity Method Investments [Line Items]
Interest receivable				$ 6,000,000	$ 4,000,000
Available for sale securities gross unrealized loss accumulated in investments	166,000,000	22,000,000
Fair value of investments in continuous unrealized loss position, less than 12 months	10,000,000,000
Available-for-sale investments (Textual) [Abstract]
Cash	539,000,000	370,000,000			526,000,000
Marketable securities	15,596,000,000	20,804,000,000			2,881,000,000
Total realized gains	158,000,000	186,000,000	191,000,000
Total realized losses	$ 83,000,000	$ 54,000,000	$ 37,000,000