Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Fair Value of Financial Instruments
Net derivative asset (liability)	$ (6,246)	$ (4,335)
Fair Value, Measurements, Recurring [Member] | Level 1
Fair Value of Financial Instruments
Net derivative asset (liability)	0	0
Fair Value, Measurements, Recurring [Member] | Level 2
Fair Value of Financial Instruments
Net derivative asset (liability)	(6,246)	(4,335)
Fair Value, Measurements, Recurring [Member] | Level 3
Fair Value of Financial Instruments
Net derivative asset (liability)	0	0
Commodity futures natural gas contracts | Fair Value, Measurements, Recurring [Member] | Level 1
Fair Value of Financial Instruments
Net derivative asset (liability)	0	0
Commodity futures natural gas contracts | Fair Value, Measurements, Recurring [Member] | Level 2
Fair Value of Financial Instruments
Net derivative asset (liability)	86	(2,202)
Commodity futures natural gas contracts | Fair Value, Measurements, Recurring [Member] | Level 3
Fair Value of Financial Instruments
Net derivative asset (liability)	0	0
Currency contracts | Fair Value, Measurements, Recurring [Member] | Level 1
Fair Value of Financial Instruments
Net derivative asset (liability)	0	0
Currency contracts | Fair Value, Measurements, Recurring [Member] | Level 2
Fair Value of Financial Instruments
Net derivative asset (liability)	(143)	245
Currency contracts | Fair Value, Measurements, Recurring [Member] | Level 3
Fair Value of Financial Instruments
Net derivative asset (liability)	0	0
Interest Rate Swap | Fair Value, Measurements, Recurring [Member] | Level 1
Fair Value of Financial Instruments
Net derivative asset (liability)	0	0
Interest Rate Swap | Fair Value, Measurements, Recurring [Member] | Level 2
Fair Value of Financial Instruments
Net derivative asset (liability)	(6,189)	(2,378)
Interest Rate Swap | Fair Value, Measurements, Recurring [Member] | Level 3
Fair Value of Financial Instruments
Net derivative asset (liability)	0	0
Total | Fair Value, Measurements, Recurring [Member]
Fair Value of Financial Instruments
Net derivative asset (liability)	(6,246)	(4,335)
Total | Commodity futures natural gas contracts | Fair Value, Measurements, Recurring [Member]
Fair Value of Financial Instruments
Net derivative asset (liability)	86	(2,202)
Total | Currency contracts | Fair Value, Measurements, Recurring [Member]
Fair Value of Financial Instruments
Net derivative asset (liability)	(143)	245
Total | Interest Rate Swap | Fair Value, Measurements, Recurring [Member]
Fair Value of Financial Instruments
Net derivative asset (liability)	$ (6,189)	$ (2,378)