Stockholders' equity - Schedule of Changes in Stockholders' Equity (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning		$ 8,907			$ 8,907
Net income	$ 1,380		$ 1,305		2,554	$ 2,522
Other comprehensive income (loss), net of taxes	(6)		8		13	16
Balance, ending	7,644				7,644
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning	1,741	1,741	1,741	$ 1,741	1,741	1,741
Net income	0	0	0	0
Dividends declared and paid	0	0	0	0
Common stock issued for stock-based awards	0	0	0	0
Stock repurchases	0	0	0	0
Stock compensation	0	0	0	0
Other comprehensive income (loss), net of taxes	0	0	0	0
Dividend equivalents on RSUs	0	0	0	0
Other			0	0
Balance, ending	1,741	1,741	1,741	1,741	1,741	1,741
Paid-in Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning	2,096	2,110	1,927	1,950	2,110	1,950
Net income	0	0	0	0
Dividends declared and paid	0	0	0	0
Common stock issued for stock-based awards	17	(77)	10	(84)
Stock repurchases	0	0	0	0
Stock compensation	69	63	67	61
Other comprehensive income (loss), net of taxes	0	0	0	0
Dividend equivalents on RSUs	0	0	0	0
Other			(1)	0
Balance, ending	2,182	2,096	2,003	1,927	2,182	2,003
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning	40,227	39,898	38,396	37,906	39,898	37,906
Net income	1,380	1,174	1,305	1,217
Dividends declared and paid	(823)	(841)	(722)	(724)
Common stock issued for stock-based awards	0	0	0	0
Stock repurchases	0	0	0	0
Stock compensation	0	0	0	0
Other comprehensive income (loss), net of taxes	0	0	0	0
Dividend equivalents on RSUs	(4)	(4)	(4)	(4)
Other			(1)	1
Balance, ending	40,780	40,227	38,974	38,396	40,780	38,974
Treasury Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning	(36,002)	(34,495)	(33,080)	(32,130)	(34,495)	(32,130)
Net income	0	0	0	0
Dividends declared and paid	0	0	0	0
Common stock issued for stock-based awards	70	223	136	235
Stock repurchases	(793)	(1,730)	(830)	(1,185)
Stock compensation	0	0	0	0
Other comprehensive income (loss), net of taxes	0	0	0	0
Dividend equivalents on RSUs	0	0	0	0
Other			(1)	0
Balance, ending	(36,725)	(36,002)	(33,775)	(33,080)	(36,725)	(33,775)
AOCI
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning	(328)	(347)	(465)	(473)	(347)	(473)
Net income	0	0	0	0
Dividends declared and paid	0	0	0	0
Common stock issued for stock-based awards	0	0	0	0
Stock repurchases	0	0	0	0
Stock compensation	0	0	0	0
Other comprehensive income (loss), net of taxes	(6)	19	8	8
Dividend equivalents on RSUs	0	0	0	0
Other			0	0
Balance, ending	$ (334)	$ (328)	$ (457)	$ (465)	$ (334)	$ (457)