Derivatives (Fair Value of Derivative Assets and Liabilities) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	$ 8	$ 1,508
Fair value, derivative liability	1,055	2,035
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	8	747
Fair value, derivative liability	993	2,035
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	0	761
Fair value, derivative liability	62	0
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	8	45
Natural Gas Contracts | Designated as Hedging Instrument | Derivative liability, current
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	45	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, current
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	57	0
Natural Gas Contracts | Not Designated as Hedging Instrument | Other long-term liabilities
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	5	0
Interest Rate Swap | Designated as Hedging Instrument | Derivative liability, current
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	852	1,928
Interest Rate Swap | Designated as Hedging Instrument | Other long-term liabilities
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	$ 96	$ 107