Derivatives (Fair Value of Derivative Assets and Liabilities) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	$ 2,359	$ 646
Fair value, derivative liability	25	716
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	2,359	646
Fair value, derivative liability	25	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	1,098	0
Interest Rate Swap | Designated as Hedging Instrument | Other assets
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	1,171	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 | Prepaid and other current assets
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	90	0
Natural Gas Contracts | Designated as Hedging Instrument | Derivative liability, current
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	0	220
Natural Gas Contracts | Designated as Hedging Instrument | Other long-term liabilities
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	25	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