Other Income (Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	$ (3,515)	$ 3,362	$ 2,880
Gain (loss) on currency transactions
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	(2,788)	775	2,641
Gain (loss) on mark-to-market natural gas contracts
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	(1,036)	1,860	(714)
Hedge ineffectiveness
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	0	0	932
Other non-operating income
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	$ 309	$ 727	$ 21