Other Income (Expense) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	$ (2,107)	$ (2,786)
Gain (loss) on currency transactions
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	(1,650)	(1,544)
Gain (loss) on mark-to-market natural gas contracts
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	0	(582)
Pension and non-pension benefits, excluding service cost
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	(340)	(526)
Other Non-Operating Income (Expense)
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	$ (117)	$ (134)