Stockholders' equity - Schedule of Changes in Stockholders' Equity (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning	$ 8,907
Net income	1,174	$ 1,217
Other comprehensive income (loss), net of taxes	19	8
Balance, ending	7,734
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning	1,741	1,741
Net income	0	0
Dividends declared and paid	0	0
Common stock issued for stock-based awards	0	0
Stock repurchases	0	0
Stock compensation	0	0
Other comprehensive income (loss), net of taxes	0	0
Dividend equivalents on RSUs	0	0
Other		0
Balance, ending	1,741	1,741
Paid-in Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning	2,110	1,950
Net income	0	0
Dividends declared and paid	0	0
Common stock issued for stock-based awards	(77)	(84)
Stock repurchases	0	0
Stock compensation	63	61
Other comprehensive income (loss), net of taxes	0	0
Dividend equivalents on RSUs	0	0
Other		0
Balance, ending	2,096	1,927
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning	39,898	37,906
Net income	1,174	1,217
Dividends declared and paid	(841)	(724)
Common stock issued for stock-based awards	0	0
Stock repurchases	0	0
Stock compensation	0	0
Other comprehensive income (loss), net of taxes	0	0
Dividend equivalents on RSUs	(4)	(4)
Other		1
Balance, ending	40,227	38,396
Treasury Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning	(34,495)	(32,130)
Net income	0	0
Dividends declared and paid	0	0
Common stock issued for stock-based awards	223	235
Stock repurchases	(1,730)	(1,185)
Stock compensation	0	0
Other comprehensive income (loss), net of taxes	0	0
Dividend equivalents on RSUs	0	0
Other		0
Balance, ending	(36,002)	(33,080)
AOCI
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning	(347)	(473)
Net income	0	0
Dividends declared and paid	0	0
Common stock issued for stock-based awards	0	0
Stock repurchases	0	0
Stock compensation	0	0
Other comprehensive income (loss), net of taxes	19	8
Dividend equivalents on RSUs	0	0
Other		0
Balance, ending	$ (328)	$ (465)