Income taxes - Provision (benefit) for income taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
U.S. Federal - Current	$ 1,110	$ 911	$ 291
Non-U.S. - Current	168	194	247
U.S. State - Current	7	9	4
Total current income tax expense (benefit)	1,285	1,114	542
U.S. Federal - Deferred	(72)	(73)	17
Non-U.S. - Deferred	14	11	33
U.S. State - Deferred	3	1	0
Total deferred income tax expense (benefit)	(55)	(61)	50
Total U.S. Federal income taxes	1,038	838	308
Total Non-U.S. income taxes	182	205	280
Total U.S. State income taxes	10	10	4
Provision for income taxes	$ 1,230	$ 1,053	$ 592