Income taxes - Reconciliation items from income tax computed at statutory federal rate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Computed tax at statutory rate	$ 1,476	$ 1,356	$ 964
Non-U.S. effective tax rates	(167)	(212)	(156)
U.S. tax benefit for manufacturing	(69)	(51)	(66)
U.S. R&D tax credit	(56)	(59)	(129)
Non-deductible expenses	13	6	13
Impact of changes to uncertain tax positions	8	3	(14)
Other	25	10	(20)
Provision for income taxes	$ 1,230	$ 1,053	$ 592