Derivatives (Effective Portion of Derivative Gain Loss) (Details) - Cash Flow Hedging - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain (loss) recognized in other comprehensive income (loss)	$ 721	$ (1,909)	$ (1,392)
Effective portion of derivative reclassified from accumulated OCI into income	(1,129)	(2,131)	573
Designated as Hedging Instrument | Natural Gas Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain (loss) recognized in other comprehensive income (loss)	721	(1,909)	(1,392)
Designated as Hedging Instrument | Natural Gas Contracts | Cost of Sales
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative reclassified from accumulated OCI into income	(1,129)	$ (2,131)	$ 573
Not Designated as Hedging Instrument | Natural Gas Contracts | Cost of Sales
Derivative Instruments, Gain (Loss) [Line Items]
Gain on Cash Flow Hedge Ineffectiveness	$ 200