Income taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Provision for income taxes	$ 216	$ 265	$ 264	$ 481	$ 501
Effective tax rate	18.30%	21.60%	20.50%
Tax reserves	$ 178			178		$ 173
Impact on effective tax rate if tax reserves were unnecessary	178			178
Accrued interest, related to income taxes in the balance sheet	34			34
Additional tax expense related to interest				3	$ 6
Reasonably possible decrease in uncertain tax positions within the next 12 months, if a re-evaluation is required	$ 100			$ 100