Employee benefit plans - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Employee benefit plans
Expense recorded	$ 12	$ 12	$ 11
Aggregate accumulated benefit obligation	583	$ 519
Pension
Employee benefit plans
Expected future contribution by the employer	20
Other Postretirement
Employee benefit plans
Expected future contribution by the employer	$ 1