Other Income (Expense) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	$ (2,260)	$ (15)
Gain (loss) on currency transactions [Member]
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	(1,544)	(730)
Gain (loss) on mark-to-market natural gas contracts
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	(582)	370
Other Non-Operating Income (Expense) [Member]
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	$ (134)	$ 345