Income taxes - Provision for income taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
U.S. federal - Current	$ 2,101	$ 1,289	$ 1,110
Non-U.S. - Current	173	238	168
U.S. state - Current	12	10	7
Total current income tax expense (benefit)	2,286	1,537	1,285
U.S. federal - Deferred	51	(122)	(72)
Non-U.S. - Deferred	61	(80)	14
U.S. state - Deferred	0	0	3
Total deferred income tax expense (benefit)	112	(202)	(55)
Total U.S. federal income taxes	2,152	1,167	1,038
Total Non-U.S. income taxes	234	158	182
Total U.S. state income taxes	12	10	10
Provision for income taxes	$ 2,398	$ 1,335	$ 1,230