Employee benefit plans - Weighted average assumptions used to determine net periodic benefit cost (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pensions Plans
Employee benefit plans
Discount rate	3.40%	3.90%	4.30%
Rate of future compensation increase	3.50%	3.50%	3.50%
Expected long-term return on plan assets	6.90%	6.80%	6.80%
Postretirement Benefit Plans
Employee benefit plans
Discount rate	3.40%	3.80%	4.10%
Rate of future compensation increase	3.60%	3.60%	3.50%