Stockholders' equity (Details 1) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Accumulated other comprehensive income
Beginning balance, Foreign currency translation	$ 22	$ 40	$ 25
Beginning balance, Cash flow hedges	3	(82)	50
Beginning balance, Available-for-sale securities	135	95	49
Beginning balance, Other	(7)	(8)	(7)
Beginning balance, Accumulated other comprehensive income	153	45	117
Other comprehensive income (loss):
Foreign currency translation adjustments	(6)	(29)	25
Unrealized gains (losses), Cash flow hedges	(51)	186	(213)
Unrealized gains (losses), Available-for-sale securities	125	155	116
Unrealized gains (losses), Other	2	1	(12)
Unrealized gains (losses), Accumulated other comprehensive income	76	342	(109)
Reclassification adjustments to income, Cash flow hedges	112	(46)	8
Reclassification adjustments to income, Available-for-sale securities	(154)	(90)	(42)
Reclassification adjustments to income, Accumulated other comprehensive income	(42)	(136)	(34)
Other	(8)		5
Other, Accumulated other comprehensive income	(8)		5
Income taxes, Foreign currency translation	5	11	(10)
Income taxes, Cash flow hedges	(21)	(55)	73
Income taxes, Available-for-sale securities	14	(25)	(28)
Income taxes, Other	0	0	6
Income taxes, Accumulated other comprehensive income (loss)	(2)	(69)	41
Ending balance, Foreign currency translation	21	22	40
Ending balance, Cash flow hedges	43	3	(82)
Ending balance, Available-for-sale securities	120	135	95
Ending balance, Other	(13)	(7)	(8)
Ending balance, Accumulated other comprehensive income	171	153	45
Foreign Currency Translation [Member]
Other comprehensive income (loss):
Foreign currency translation adjustments	$ (6)	$ (29)	$ 25