Employee benefit plans - Components of net periodic benefit cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension
Employee benefit plans
Interest cost	$ 17	$ 21	$ 19
Expected return on plan assets	(24)	(24)	(27)
Actual return on plan assets	40	70
Amortization of net loss (gain)	1
Expenses paid	2	2	2
Net periodic benefit cost	(4)	(1)	(6)
Other Postretirement
Employee benefit plans
Interest cost		1	1
Amortization of net loss (gain)		$ (1)	$ (1)
Settlement loss recognized	(4)
Net periodic benefit cost	$ (4)