Income taxes - Income tax expense components (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income tax expense:
Federal	$ 9,253	$ 6,447	$ 8,155
State	578	560	258
Foreign	701	928	538
Total income taxes	10,532	7,935	8,951
Income tax expense:
Current	5,426	3,302	5,168
Deferred	5,106	4,633	3,783
Total income taxes	$ 10,532	$ 7,935	$ 8,951