Stockholders' equity (Details Textual) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Amount available for stock repurchases under a board approved stock repurchase plan	$ 4,400												$ 4,400
Common stock, dividends paid per share (in usd per share)	$ 1.15	$ 1.15	$ 1.15	$ 1.15	$ 1.00	$ 1	$ 1	$ 1	$ 0.79	$ 0.79	$ 0.79	$ 0.79
Common stock, dividends declared per share (in usd per share)	$ 1.32
Income taxes expense or (benefit) for unrealized gains and losses for cash flow hedges													68	$ (68)	$ 53
Income tax expense or (benefit) reclassification adjustments to income for cash flow hedges													(226)	46	(53)
Income taxes expense or (benefit) for unrealized gains and losses for available-for-sale securities													9	(9)	0
Income tax expense or (benefit) for reclassification adjustments to income for available-for-sale securities													$ (8)	$ 11	$ 18
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