Employee benefit plans - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee benefit plans
Expense recorded	$ 17,000,000	$ 15,000,000	$ 14,000,000
Pensions Plans
Employee benefit plans
Gain on settlement			$ (3,000,000)
Aggregate accumulated benefit obligation	593,000,000	652,000,000
Accumulated benefit obligation	536,000,000	587,000,000
Expected future contribution by the employer	14,000,000
Postretirement Benefit Plans
Employee benefit plans
Gain on settlement		24,000,000
Accumulated benefit obligation	$ 32,000,000	$ 39,000,000
Assumed health care cost trend rate	6.50%
Decrease in health care cost trend rate	$ 4.5
Expected future contribution by the employer	$ 2,000,000