Fair Value (Derivative Assets and Liabilities by Hierarchy) (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Level One
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net derivative asset (liability)	$ 0	$ 0
Level Two
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net derivative asset (liability)	(70)	(527)
Level Three
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net derivative asset (liability)	0	0
Commodity Futures Natural Gas Contracts | Level One
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net derivative asset (liability)	0	0
Commodity Futures Natural Gas Contracts | Level Two
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net derivative asset (liability)	(503)	1,508
Commodity Futures Natural Gas Contracts | Level Three
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net derivative asset (liability)	0	0
Interest Rate Swap | Level One
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net derivative asset (liability)	0	0
Interest Rate Swap | Level Two
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net derivative asset (liability)	433	(2,035)
Interest Rate Swap | Level Three
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net derivative asset (liability)	0	0
Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net derivative asset (liability)	(70)	(527)
Total | Commodity Futures Natural Gas Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net derivative asset (liability)	(503)	1,508
Total | Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net derivative asset (liability)	$ 433	$ (2,035)