Stockholders' equity (Details Textual) - USD ($) $ / shares in Units, shares in Millions		1 Months Ended						3 Months Ended						9 Months Ended
	Mar. 08, 2018	Oct. 31, 2018	Sep. 30, 2018	Jul. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Apr. 30, 2018	Jan. 31, 2018
Equity [Abstract]
Stock repurchase program, additional authorized amount																$ 5,000,000,000.0	$ 10,000,000,000.0
Number of shares repurchased	52.1							8.7	18.2	56.4	4.4	6.2	3.4	83.4	14.0
Stock repurchase program, tender offer amount										$ 10,000,000,000
Amount available for stock repurchases under a board approved stock repurchase plan			$ 3,700,000,000					$ 3,700,000,000						$ 3,700,000,000
Dividends paid per share (in usd per share)			$ 1.32		$ 1.32	$ 1.32		$ 1.32			$ 1.15			$ 3.96	$ 3.45
Subsequent Event [Line Items]
Dividends declared per share (in usd per share)				$ 1.32		$ 1.32	$ 1.32
Subsequent Event [Member]
Subsequent Event [Line Items]
Dividends declared per share (in usd per share)		$ 1.32