Income taxes - Provision for Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Tax Disclosure [Abstract]
Taxes calculated using the estimated annual effective tax rate	$ 164	$ 238	$ 330	$ 458
Discrete tax items	(265)	(29)	(381)	(89)
Provision for income taxes	$ (101)	$ 209	$ (51)	$ 369
Actual effective tax rate	(8.00%)	14.00%	(2.00%)	13.00%