Derivatives Derivatives (Summary of gains (losses) recognized in Statement of Operations and AOCI) (Details) - Cash Flow Hedging - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Designated as Hedging Instrument
Derivative [Line Items]
Effective portion of derivative gain (loss) reclassified from accumulated other comprehensive loss into income (loss)	$ (341)	$ (518)
Not Designated as Hedging Instrument
Derivative [Line Items]
Gain (loss) on contracts where hedge accounting was not elected	0	(583)
Other Comprehensive Income (Loss) | Designated as Hedging Instrument
Derivative [Line Items]
Effective portion of derivative gain (loss) recognized in other comprehensive income (loss)	1,464	(266)
Other Comprehensive Income (Loss) | Designated as Hedging Instrument | Natural Gas Contracts
Derivative [Line Items]
Effective portion of derivative gain (loss) recognized in other comprehensive income (loss)	211	(470)
Other Comprehensive Income (Loss) | Designated as Hedging Instrument | Interest Rate Swap
Derivative [Line Items]
Effective portion of derivative gain (loss) recognized in other comprehensive income (loss)	1,253	204
Cost of Sales | Designated as Hedging Instrument | Natural Gas Contracts
Derivative [Line Items]
Effective portion of derivative gain (loss) reclassified from accumulated other comprehensive loss into income (loss)	(198)	67
Interest Expense | Designated as Hedging Instrument | Interest Rate Swap
Derivative [Line Items]
Effective portion of derivative gain (loss) reclassified from accumulated other comprehensive loss into income (loss)	(143)	(585)
Other Non-Operating Income (Expense) | Not Designated as Hedging Instrument | Natural Gas Contracts
Derivative [Line Items]
Gain (loss) on contracts where hedge accounting was not elected	$ 0	$ (583)