Other Income (Expense) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	$ (644)	$ 802	$ (2,904)	$ 787
Gain (loss) on currency transactions
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	(691)	6	(2,235)	(724)
Gain (loss) on mark-to-market natural gas contracts
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	(236)	769	(819)	1,139
Other Non-Operating Income (Expense)
Component of Other Income (Expense), Nonoperating [Line Items]
Other income (expense)	$ 283	$ 27	$ 150	$ 372