Other Finance Income (Costs) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Other Nonoperating Income (Expense) [Abstract]
Pension interest cost	$ (693)	$ (757)
Expected return on pension plan assets	929	956
Foreign currency translation on senior bank debt	2,855
Foreign currency remeasurement on hedging instrument	(1,927)
Other finance income (costs)	$ 1,164	$ 199