Fair Value (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Level 1
Fair Value of Financial Instruments
Net derivative asset (liability)	$ 0	$ 0
Level 2
Fair Value of Financial Instruments
Net derivative asset (liability)	2,334	(70)
Level 3
Fair Value of Financial Instruments
Net derivative asset (liability)	0	0
Commodity futures natural gas contracts | Level 1
Fair Value of Financial Instruments
Net derivative asset (liability)	0	0
Commodity futures natural gas contracts | Level 2
Fair Value of Financial Instruments
Net derivative asset (liability)	65	(503)
Commodity futures natural gas contracts | Level 3
Fair Value of Financial Instruments
Net derivative asset (liability)	0	0
Interest Rate Swap | Level 1
Fair Value of Financial Instruments
Net derivative asset (liability)	0	0
Interest Rate Swap | Level 2
Fair Value of Financial Instruments
Net derivative asset (liability)	2,269	433
Interest Rate Swap | Level 3
Fair Value of Financial Instruments
Net derivative asset (liability)	0	0
Total
Fair Value of Financial Instruments
Net derivative asset (liability)	2,334	(70)
Total | Commodity futures natural gas contracts
Fair Value of Financial Instruments
Net derivative asset (liability)	65	(503)
Total | Interest Rate Swap
Fair Value of Financial Instruments
Net derivative asset (liability)	$ 2,269	$ 433