Earnings per share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income (Numerator):
Net income for basic and diluted EPS	$ 1,859	$ 2,021	$ 6,466	$ 6,243
Shares (Denominator):
Weighted-average shares for basic EPS (in shares)	645	728	669	733
Effect of dilutive securities (in shares)	4	5	4	5
Weighted-average shares for diluted EPS (in shares)	649	733	673	738
Basic EPS (in usd per share)	$ 2.88	$ 2.78	$ 9.67	$ 8.52
Diluted EPS (in usd per share)	$ 2.86	$ 2.76	$ 9.61	$ 8.46