Income taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Income Tax Disclosure [Abstract]
Provision for income taxes	$ 265	$ 373	$ 237
Effective tax rate	21.60%	20.50%	19.90%
Tax reserves	$ 176	$ 173
Impact on effective tax rate if tax reserves were unnecessary	176
Accrued interest, related to income taxes in the balance sheet	35
Additional tax expense related to interest	4		$ 2
Reasonably possible decrease in uncertain tax positions within the next 12 months, if a re-evaluation is required	$ 100