Income taxes - Provision (benefit) for income taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
U.S. Federal - Current	$ 911	$ 291	$ (108)
Non-U.S. - Current	194	247	156
U.S. State - Current	9	4	(2)
Total current income tax expense (benefit)	1,114	542	46
U.S. Federal - Deferred	(73)	17	65
Non-U.S. - Deferred	11	33	65
U.S. State - Deferred	1	0	0
Total deferred income tax expense (benefit)	(61)	50	130
Total U.S. Federal income taxes	838	308	(43)
Total Non-U.S. income taxes	205	280	221
Total U.S. State income taxes	10	4	(2)
Provision for income taxes	$ 1,053	$ 592	$ 176