Other Income (Expense) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	$ (1,453)	$ 193	$ (980)	$ (3,445)
Gain (loss) on currency transactions
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	(1,294)	548	(282)	(1,687)
(Loss) on mark-to-market natural gas contracts
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	0	(4)	0	(823)
Pension and non-pension benefits, excluding service cost
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	(295)	(428)	(878)	(1,162)
Other Non-Operating Income (Expense)
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	$ 136	$ 77	$ 180	$ 227