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Summary of Significant Accounting Policies - Schedule of Derivative Instruments (Detail) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Derivative [Line Items]    
Net Fair Value Presented in the Consolidated Balance Sheet, Asset Derivatives - current   $ 1,318
Net Fair Value Presented in the Consolidated Balance Sheet, Liabilities Derivatives - current $ 230  
Natural Gas Price Derivatives | Derivative Financial Instruments – Current    
Derivative [Line Items]    
Asset Derivatives, Fair Value 79 1,318
Gross Amounts Offset in the Consolidated Balance Sheet, Assets Derivatives (79) 0
Net Fair Value Presented in the Consolidated Balance Sheet, Asset Derivatives - current 0 $ 1,318
Liability Derivatives, Fair Value 309  
Gross Amounts Offset in the Consolidated Balance Sheet, Liabilities Derivatives (79)  
Net Fair Value Presented in the Consolidated Balance Sheet, Liabilities Derivatives - current $ 230