Derivatives (Fair Value of Derivative Assets and Liabilities) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	$ 1,829	$ 646
Fair value, derivative liability	94	716
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	1,829	646
Fair value, derivative liability	94	440
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	0	276
Interest Rate Swap | Designated as Hedging Instrument | Prepaid and other current assets
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	567	0
Interest Rate Swap | Designated as Hedging Instrument | Other assets
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	1,262	646
Interest Rate Swap | Designated as Hedging Instrument | Derivative liability, current
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	0	213
Natural Gas Contracts | Designated as Hedging Instrument | Derivative liability, current
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	87	220
Natural Gas Contracts | Designated as Hedging Instrument | Other long-term liabilities
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	7	7
Natural Gas Contracts | Not Designated as Hedging Instrument | Derivative liability, current
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	0	264
Natural Gas Contracts | Not Designated as Hedging Instrument | Other long-term liabilities
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	$ 0	$ 12