Income taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Provision for income taxes	$ 213	$ 216	$ 246	$ 694	$ 747
Effective tax rate	18.40%	18.30%	19.10%
Tax reserves	$ 85			85		$ 173
Impact on effective tax rate if tax reserves were unnecessary	85			85
Accrued interest, related to income taxes in the balance sheet	23			23
Additional tax expense related to interest				5	$ 9
Reasonably possible decrease in uncertain tax positions within the next 12 months, if a re-evaluation is required	$ 15			$ 15