Stockholders' equity (Details Textual) (USD $)	1 Months Ended																					3 Months Ended												12 Months Ended
	Dec. 13, 2013	Dec. 06, 2013	Oct. 16, 2013	Sep. 06, 2013	Jul. 26, 2013	Jun. 07, 2013	Mar. 07, 2013	Mar. 06, 2013	Dec. 13, 2012	Dec. 07, 2012	Oct. 10, 2012	Sep. 07, 2012	Jul. 19, 2012	Jun. 07, 2012	Mar. 15, 2012	Mar. 07, 2012	Dec. 15, 2011	Dec. 08, 2011	Oct. 13, 2011	Sep. 08, 2011	Jul. 28, 2011	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Stock Repurchase Program [Line Items]
Stock repurchases (in shares)																						0	0	0	9,100,000	14,200,000	9,700,000	17,400,000	21,000,000	86,000,000	45,400,000	12,900,000	0	9,100,000	62,300,000	144,300,000
Amount available for stock repurchases under a board approved stock repurchase plan																																		$ 1,600,000,000
Common stock, dividends declared per share (in usd per share)	$ 0.61		$ 0.47		$ 0.47			$ 0.47	$ 0.47		$ 0.36		$ 0.36		$ 0.36		$ 0.36		$ 0.28		$ 0.28
Dividends paid per share (in usd per share)		$ 0.47		$ 0.47		$ 0.47	$ 0.47			$ 0.36		$ 0.36		$ 0.36		$ 0.36		$ 0.28		$ 0.28
Income taxes expense or (benefit) for unrealized gains and losses for cash flow hedges																																		34,000,000	8,000,000	(20,000,000)
Income tax expense or (benefit) reclassification adjustments to income for cash flow hedges																																		(33,000,000)	(49,000,000)	41,000,000
Income taxes expense or (benefit) for unrealized gains and losses for available-for-sale securities																																		(105,000,000)	87,000,000	45,000,000
Income tax expense or (benefit) for reclassification adjustments to income for available-for-sale securities																																		(28,000,000)	(49,000,000)	(59,000,000)
Preferred stock shares authorized (in shares)																						5,000,000												5,000,000
Preferred stock, par value (in usd per share)																						$ 0.0001												$ 0.0001
Preferred stock, shares issued (in shares)																						0				0								0	0
Preferred stock, shares outstanding (in shares)																						0				0								0	0
Modified Dutch Auction Tender Offer [Member]
Stock Repurchase Program [Line Items]
Stock repurchases (in shares)																																				83,300,000
Stock repurchased price per share (in usd per share)																																				$ 60.08
Stock repurchases																																				$ 5,000,000,000