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Summary of Significant Accounting Policies - Summary of Outstanding Derivative Financial Instruments (Detail) - Natural Gas Swap Agreements
 3 Months Ended
Mar. 31, 2018
MMBTU
$ / MMBTU
Derivative [Line Items]  
Commodity Derivatives Weighted Average Contract Price | $ / MMBTU 3.00
Commodity Derivatives Contract Volume | MMBTU 20,400,000
Commodity Derivatives Beginning of Remaining Contract Period Apr. 01, 2018
Commodity Derivatives End of Remaining Contract Period Mar. 31, 2019