Stockholders' equity (Details Textual) - USD ($) $ / shares in Units, $ in Billions			3 Months Ended				9 Months Ended
	Oct. 24, 2017	Sep. 30, 2013	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Equity, Class of Treasury Stock [Line Items]
Amount available for stock repurchases under a board approved stock repurchase plan			$ 1.7				$ 1.7
Dividends paid per share (in usd per share)		$ 1.15	$ 1.15	$ 1.15	$ 1.15	$ 1	$ 3.45	$ 3
Subsequent event [Member]
Equity, Class of Treasury Stock [Line Items]
Amount available for stock repurchases under a board approved stock repurchase plan	$ 5.0
Dividends declared per share (in usd per share)	$ 1.15