Income taxes - Income tax expense components (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income tax expense:
Federal	$ 10,596	$ 19,069	$ (1,613)
State	1,086	625	175
Foreign	758	1,210	1,117
Total income taxes	12,440	20,904	(321)
Income tax expense:
Current	5,052	5,818	5,176
Deferred	7,388	15,086	(5,497)
Total income taxes	$ 12,440	$ 20,904	$ (321)