Employee benefit plans - Components of net periodic benefit cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pensions Plans
Employee benefit plans
Service cost	$ 4	$ 3	$ 2
Interest cost	22	24	26
Actual return on plan assets	(32)	(29)	(31)
Amortization of net loss (gain)	1	1	2
Settlement loss recognized			3
Expenses paid	2	3	4
Net periodic benefit cost	(3)	2	6
Postretirement Benefit Plans
Employee benefit plans
Service cost			1
Interest cost	2	2	2
Amortization of prior service credit		(2)	(2)
Amortization of net loss (gain)	(1)
Curtailment gain		(26)
Settlement loss recognized		(24)
Net periodic benefit cost	$ 1	$ (26)	$ 1