Pensions - Schedule of significant weighted average assumptions (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Retirement Benefits [Abstract]
Discount rate for calculating net periodic benefit costs (as a percent)	2.50%	2.70%	3.85%
Expected long-term rate of return on plan assets (as a percent)	3.96%	3.87%	4.90%
Discount rate for calculating benefit obligation (as a percent)	2.80%	2.50%