Derivatives (Fair Value of Derivative Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	$ 1,508	$ 245
Fair value, derivative liability	2,035	4,580
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	747	0
Fair value, derivative liability	2,035	3,481
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	761	245
Fair value, derivative liability	0	1,099
Natural Gas Contracts | Designated as Hedging Instrument | Prepaid and other current assets
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	702	0
Natural Gas Contracts | Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	45	0
Natural Gas Contracts | Designated as Hedging Instrument | Derivative Liability - Current
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	0	1,069
Natural Gas Contracts | Designated as Hedging Instrument | Other Long-Term Liabilities
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	0	34
Natural Gas Contracts | Not Designated as Hedging Instrument | Prepaid and other current assets
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	732	0
Natural Gas Contracts | Not Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	29	0
Natural Gas Contracts | Not Designated as Hedging Instrument | Derivative Liability - Current
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	0	1,064
Natural Gas Contracts | Not Designated as Hedging Instrument | Other Long-Term Liabilities
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	0	35
Interest Rate Swap - Fixed | Designated as Hedging Instrument | Derivative Liability - Current
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	1,928	2,132
Interest Rate Swap - Fixed | Designated as Hedging Instrument | Other Long-Term Liabilities
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	107	246
Currency Contracts | Not Designated as Hedging Instrument | Prepaid and other current assets
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	$ 0	$ 245