Other Income (Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	$ (2,764)	$ (5,306)
Gain (loss) on currency transactions
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	(1,454)	(2,788)
(Loss) on mark-to-market natural gas contracts
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	0	(1,036)
Pension and non-pension benefits, excluding service cost
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	(1,232)	(1,791)
Other non-operating income (expense)
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	$ (78)	$ 309