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
	Mar. 31, 2020	Mar. 31, 2019
Basic EPS:
Net income	$ 1,174	$ 1,217
Income allocated to RSUs	(6)	(8)
Income allocated to common stock for basic EPS	$ 1,168	$ 1,209
Weighted average number of shares outstanding, basic (in shares)	931	939
Basic (in dollars per share)	$ 1.25	$ 1.29
Dilutive effect of stock compensation plans (in shares)	12	17
Diluted EPS:
Net income	$ 1,174	$ 1,217
Income allocated to RSUs	(6)	(8)
Income allocated to common stock for diluted EPS	$ 1,168	$ 1,209
Weighted average number of shares outstanding, diluted (in shares)	943	956
Diluted (in dollars per share)	$ 1.24	$ 1.26