Derivatives (Fair Value of Derivative Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	$ 646	$ 1,508
Fair value, derivative liability	716	2,035
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	646	747
Fair value, derivative liability	440	2,035
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	0	761
Fair value, derivative liability	276	0
Interest Rate Swap | Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	646	0
Interest Rate Swap | Designated as Hedging Instrument | Derivative Liability
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	213	1,928
Interest Rate Swap | Designated as Hedging Instrument | Other Long-Term Liabilities
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	0	107
Natural Gas Contracts | Designated as Hedging Instrument | Prepaid and other current assets
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	0	702
Natural Gas Contracts | Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	0	45
Natural Gas Contracts | Designated as Hedging Instrument | Derivative Liability
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	220	0
Natural Gas Contracts | Designated as Hedging Instrument | Other Long-Term Liabilities
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	7	0
Natural Gas Contracts | Not Designated as Hedging Instrument | Prepaid and other current assets
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	0	732
Natural Gas Contracts | Not Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	0	29
Natural Gas Contracts | Not Designated as Hedging Instrument | Derivative Liability
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	264	0
Natural Gas Contracts | Not Designated as Hedging Instrument | Other Long-Term Liabilities
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	$ 12	$ 0