Employee benefit plans - Components of net periodic benefit (credits) cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension
Employee benefit plans
Interest cost	$ 21	$ 19	$ 21
Expected return on plan assets	(24)	(27)	(24)
Actual return on plan assets	70	(19)
Expenses paid	2	2	2
Net periodic benefit (credit) cost	(1)	(6)	(1)
Other Postretirement
Employee benefit plans
Interest cost	1	1	2
Amortization of prior service credit			(2)
Amortization of net loss (gain)	$ (1)	$ (1)	(1)
Curtailment gain			(26)
Net periodic benefit (credit) cost			$ (27)