Income taxes - Provision (benefit) for income taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
U.S. federal - Current	$ 1,289	$ 1,110	$ 911
Non-U.S. - Current	238	168	194
U.S. state - Current	10	7	9
Total current income tax expense (benefit)	1,537	1,285	1,114
U.S. federal - Deferred	(122)	(72)	(73)
Non-U.S. - Deferred	(80)	14	11
U.S. state - Deferred	0	3	1
Total deferred income tax expense (benefit)	(202)	(55)	(61)
Total U.S. federal income taxes	1,167	1,038	838
Total Non-U.S. income taxes	158	182	205
Total U.S. state income taxes	10	10	10
Provision for income taxes	$ 1,335	$ 1,230	$ 1,053