Employee benefit plans - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Employee benefit plans
Expense recorded			$ 16
Pension
Employee benefit plans
Aggregate accumulated benefit obligation	$ 624		624	$ 583
Employer contributions	15	$ 3	34	$ 12
Expected future contribution by the employer	$ 16		$ 16