Stockholders’ equity - Schedule of Changes in Stockholders' Equity (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning			$ 8,994				$ 8,994
Net income	$ 1,425			$ 1,570			3,947	$ 4,341
Other comprehensive income (loss), net of taxes	14			17			30	30
Balance, ending	8,985						8,985
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
Cumulative effect of accounting changes						0
Other	0	0	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	2,003	1,927	1,950	1,867	1,770	1,776	1,950	1,776
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	8	10	(84)	6	22	(75)
Stock repurchases	0	0	0	0	0	0
Stock compensation	48	67	61	46	74	70
Other comprehensive income (loss), net of taxes	0	0	0	0	0	0
Dividend equivalents on RSUs	0	0	0	0	0	0
Cumulative effect of accounting changes						0
Other	(1)	(1)	0	(1)	1	(1)
Balance, ending	2,058	2,003	1,927	1,918	1,867	1,770	2,058	1,918
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning	38,974	38,396	37,906	36,413	35,619	34,662	37,906	34,662
Net income	1,425	1,305	1,217	1,570	1,405	1,366
Dividends declared and paid	(721)	(722)	(724)	(602)	(606)	(611)
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	(4)	(4)	(4)	(4)	(5)	(4)
Cumulative effect of accounting changes						206
Other	0	(1)	1	1	0	0
Balance, ending	39,674	38,974	38,396	37,378	36,413	35,619	39,674	37,378
Treasury Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning	(33,775)	(33,080)	(32,130)	(29,016)	(28,096)	(27,458)	(32,130)	(27,458)
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	186	136	235	49	80	253
Stock repurchases	(456)	(830)	(1,185)	(1,200)	(1,000)	(891)
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
Cumulative effect of accounting changes						0
Other	0	(1)	0	0	0	0
Balance, ending	(34,045)	(33,775)	(33,080)	(30,167)	(29,016)	(28,096)	(34,045)	(30,167)
AOCI
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning	(457)	(465)	(473)	(371)	(392)	(384)	(473)	(384)
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	14	8	8	17	21	(8)
Dividend equivalents on RSUs	0	0	0	0	0	0
Cumulative effect of accounting changes						0
Other	0	0	0	0	0	0
Balance, ending	$ (443)	$ (457)	$ (465)	$ (354)	$ (371)	$ (392)	$ (443)	$ (354)