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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Basic EPS:
Net income	$ 1,425	$ 1,570	$ 3,947	$ 4,341
Income allocated to RSUs	(9)	(12)	(25)	(34)
Income allocated to common stock for basic EPS	$ 1,416	$ 1,558	$ 3,922	$ 4,307
Weighted average number of shares outstanding, basic (in shares)	935	969	937	976
Basic earnings per common share (in dollars per share)	$ 1.51	$ 1.61	$ 4.19	$ 4.41
Dilutive effect of stock compensation plans (in shares)	15	20	16	21
Diluted EPS:
Net income	$ 1,425	$ 1,570	$ 3,947	$ 4,341
Income allocated to RSUs	(8)	(11)	(25)	(34)
Income allocated to common stock for diluted EPS	$ 1,417	$ 1,559	$ 3,922	$ 4,307
Weighted average number of shares outstanding, diluted (in shares)	950	989	953	997
Diluted earnings per common share (in dollars per share)	$ 1.49	$ 1.58	$ 4.12	$ 4.32