Stockholders' equity (Details Textual) - USD ($)	1 Months Ended						3 Months Ended		9 Months Ended
	Oct. 31, 2016	Sep. 30, 2016	Jul. 29, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Subsequent Event [Line Items]
Amount available for stock repurchases under a board approved stock repurchase plan		$ 2,900,000,000					$ 2,900,000,000		$ 2,900,000,000
Common stock, dividends declared per share (in usd per share)			$ 1.00		$ 1.00	$ 1.00
Dividends paid per share (in usd per share)		$ 1		$ 1.00	$ 1.00		$ 1.0	$ 0.79	$ 3.0	$ 2.37
Subsequent Event [Member]
Subsequent Event [Line Items]
Amount authorized for stock repurchase	$ 5,000,000,000.0
Common stock, dividends declared per share (in usd per share)	$ 1.00