Earnings per share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income (Numerator):
Net income for basic and diluted EPS	$ 2,311	$ 2,071
Shares (Denominator):
Weighted-average shares for basic EPS	707	737
Effect of dilutive securities	4	4
Weighted-average shares for diluted EPS	711	741
Basic EPS (in usd per share)	$ 3.27	$ 2.81
Diluted EPS (in usd per share)	$ 3.25	$ 2.79