Derivatives (Fair Value of Derivative Assets and Liabilities) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	$ 0	$ 1,690
Fair value, derivative liability	14,384	5,713
Interest Rate Swaps | Designated as Hedging Instrument | Prepaid and other current assets
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	0	1,425
Interest Rate Swaps | Designated as Hedging Instrument | Other long-term liabilities
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	12,363	5,713
Interest Rate Swaps | Designated as Hedging Instrument | Accrued liabilities
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	1,080	0
Natural Gas Contracts | Designated as Hedging Instrument | Prepaid and other current assets
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	0	226
Natural Gas Contracts | Designated as Hedging Instrument | Other assets
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	0	39
Natural Gas Contracts | Designated as Hedging Instrument | Other long-term liabilities
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	87	0
Natural Gas Contracts | Designated as Hedging Instrument | Accrued liabilities
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	$ 854	$ 0