Stockholders' equity - Schedule of Changes in Stockholders' Equity (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning			$ 8,907				$ 8,907
Net income	$ 1,353			$ 1,425			3,907	$ 3,947
Other comprehensive income (loss), net of taxes	(1)			14			12	30
Balance, ending	8,325						8,325
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning	1,741	$ 1,741	1,741	1,741	$ 1,741	$ 1,741	1,741	1,741
Net income	0	0	0	0	0	0
Dividends declared and paid	0	0	0	0	0	0
Common stock issued for stock-based awards	0	0	0	0	0	0
Stock repurchases	0	0	0	0	0	0
Stock compensation	0	0	0	0	0	0
Other comprehensive income (loss), net of taxes	0	0	0	0	0	0
Dividend equivalents on RSUs	0	0	0	0	0	0
Other	0			0	0	0
Balance, ending	1,741	1,741	1,741	1,741	1,741	1,741	1,741	1,741
Paid-in Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning	2,182	2,096	2,110	2,003	1,927	1,950	2,110	1,950
Net income	0	0	0	0	0	0
Dividends declared and paid	0	0	0	0	0	0
Common stock issued for stock-based awards	26	17	(77)	8	10	(84)
Stock repurchases	0	0	0	0	0	0
Stock compensation	50	69	63	48	67	61
Other comprehensive income (loss), net of taxes	0	0	0	0	0	0
Dividend equivalents on RSUs	0	0	0	0	0	0
Other	(1)			(1)	(1)	0
Balance, ending	2,257	2,182	2,096	2,058	2,003	1,927	2,257	2,058
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning	40,780	40,227	39,898	38,974	38,396	37,906	39,898	37,906
Net income	1,353	1,380	1,174	1,425	1,305	1,217
Dividends declared and paid	(825)	(823)	(841)	(721)	(722)	(724)
Common stock issued for stock-based awards	0	0	0	0	0	0
Stock repurchases	0	0	0	0	0	0
Stock compensation	0	0	0	0	0	0
Other comprehensive income (loss), net of taxes	0	0	0	0	0	0
Dividend equivalents on RSUs	(3)	(4)	(4)	(4)	(4)	(4)
Other	0			0	(1)	1
Balance, ending	41,305	40,780	40,227	39,674	38,974	38,396	41,305	39,674
Treasury Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning	(36,725)	(36,002)	(34,495)	(33,775)	(33,080)	(32,130)	(34,495)	(32,130)
Net income	0	0	0	0	0	0
Dividends declared and paid	0	0	0	0	0	0
Common stock issued for stock-based awards	97	70	223	186	136	235
Stock repurchases	(15)	(793)	(1,730)	(456)	(830)	(1,185)
Stock compensation	0	0	0	0	0	0
Other comprehensive income (loss), net of taxes	0	0	0	0	0	0
Dividend equivalents on RSUs	0	0	0	0	0	0
Other	0			0	(1)	0
Balance, ending	(36,643)	(36,725)	(36,002)	(34,045)	(33,775)	(33,080)	(36,643)	(34,045)
AOCI
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning	(334)	(328)	(347)	(457)	(465)	(473)	(347)	(473)
Net income	0	0	0	0	0	0
Dividends declared and paid	0	0	0	0	0	0
Common stock issued for stock-based awards	0	0	0	0	0	0
Stock repurchases	0	0	0	0	0	0
Stock compensation	0	0	0	0	0	0
Other comprehensive income (loss), net of taxes	(1)	(6)	19	14	8	8
Dividend equivalents on RSUs	0	0	0	0	0	0
Other	0			0	0	0
Balance, ending	$ (335)	$ (334)	$ (328)	$ (443)	$ (457)	$ (465)	$ (335)	$ (443)