Income taxes - Provision for Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Taxes calculated using the estimated annual effective tax rate	$ 231	$ 257	$ 561	$ 715
Discrete tax items	3	(102)	(378)	(191)
Provision for income taxes	$ 234	$ 155	$ 183	$ 524
Actual effective tax rate	15.00%	10.00%	4.00%	12.00%