Basis of presentation and significant accounting policies and practices - Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Basic EPS:
Net income	$ 1,353	$ 1,425	$ 3,907	$ 3,947
Income allocated to RSUs	(6)	(9)	(19)	(25)
Income allocated to common stock for basic EPS	$ 1,347	$ 1,416	$ 3,888	$ 3,922
Weighted average number of shares outstanding, basic (in shares)	917	935	921	937
Basic (in dollars per share)	$ 1.47	$ 1.51	$ 4.22	$ 4.19
Dilutive effect of stock compensation plans (in shares)	12	15	12	16
Diluted EPS:
Net income	$ 1,353	$ 1,425	$ 3,907	$ 3,947
Income allocated to RSUs	(6)	(8)	(19)	(25)
Income allocated to common stock for diluted EPS	$ 1,347	$ 1,417	$ 3,888	$ 3,922
Weighted average number of shares outstanding, diluted (in shares)	929	950	933	953
Diluted (in dollars per share)	$ 1.45	$ 1.49	$ 4.17	$ 4.12