Fair Value of Financial Instruments (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Measurements, Recurring \| Commodity Contract \| Natural gas swap contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset (liability)	$ (178,000)	$ 1,644,000
Fair Value, Measurements, Recurring \| Commodity Contract \| Natural gas swap contracts \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset (liability)	0	0
Fair Value, Measurements, Recurring \| Commodity Contract \| Natural gas swap contracts \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset (liability)	(178,000)	1,644,000
Fair Value, Measurements, Recurring \| Commodity Contract \| Natural gas swap contracts \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset (liability)	0	$ 0
Senior secured notes \| Senior Secured Notes Due 2024 \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of long-term debt	471,400,000
Citibank \| Revolving Credit Agreement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amount outstanding	0
Citibank \| Revolving Credit Agreement \| Letter of credit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amount outstanding	$ 4,600,000.0

Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

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Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
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