Earnings per share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Income (Numerator):
Net income for basic and diluted EPS	$ 1,968	$ 2,179	$ 1,992	$ 1,859	$ 2,296	$ 2,311	$ 6,139	$ 6,466
Shares (Denominator):
Weighted-average shares for basic EPS (in shares)	599			645			609	669
Effect of dilutive securities (in shares)	3			4			4	4
Weighted-average shares for diluted EPS (in shares)	602			649			613	673
Basic EPS (in usd per share)	$ 3.29			$ 2.88			$ 10.08	$ 9.67
Diluted EPS (in usd per share)	$ 3.27			$ 2.86			$ 10.01	$ 9.61