Earnings per share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income (Numerator):
Net income for basic and diluted EPS	$ 2,021	$ 2,017	$ 6,243	$ 5,787
Shares (Denominator):
Weighted-average shares for basic EPS	728	747	733	752
Effect of dilutive securities	5	6	5	6
Weighted-average shares for diluted EPS	733	753	738	758
Basic EPS (in usd per share)	$ 2.78	$ 2.70	$ 8.52	$ 7.70
Diluted EPS (in usd per share)	$ 2.76	$ 2.68	$ 8.46	$ 7.63