Income taxes - Income tax expense components (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income tax expense:
Federal	$ (23,427)	$ 7,796	$ 9,253
State	894	556	578
Foreign	1,018	888	701
Total income taxes	(21,515)	9,240	10,532
Income tax expense:
Current	3,299	6,565	5,426
Deferred	(24,814)	2,675	5,106
Total income taxes	$ (21,515)	$ 9,240	$ 10,532