Income taxes - Provision for income taxes (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Tax Disclosure [Abstract]
Taxes calculated using the estimated annual effective tax rate	$ 458	$ 325	$ 840	$ 606
Discrete tax items	(28)	(42)	(152)	(84)
Provision for income taxes	$ 430	$ 283	$ 688	$ 522
Estimated annual effective tax rate	31.00%	30.00%	31.00%	30.00%
Actual effective tax rate	29.00%	26.00%	25.00%	25.00%