Derivatives (Fair Value of Derivative Assets and Liabilities) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	$ 863	$ 1,690
Fair value, derivative liability	8,883	5,713
Interest Rate Swaps | Designated as Hedging Instrument | Prepaid and other current assets
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	672	1,425
Interest Rate Swaps | Designated as Hedging Instrument | Other long-term liabilities
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	8,793	5,713
Natural Gas Contracts | Designated as Hedging Instrument | Prepaid and other current assets
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	142	226
Natural Gas Contracts | Designated as Hedging Instrument | Other assets
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	49	39
Natural Gas Contracts | Designated as Hedging Instrument | Other long-term liabilities
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	6	0
Natural Gas Contracts | Designated as Hedging Instrument | Accrued liabilities
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	$ 84	$ 0