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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 |

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