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Natural Gas Derivative Instruments - Additional Information (Detail)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2016
USD ($)
$ / MMBTU
Sep. 30, 2015
USD ($)
$ / MMBTU
Sep. 30, 2016
USD ($)
MMBTU
$ / MMBTU
Sep. 30, 2015
USD ($)
$ / MMBTU
Natural gas derivative contract        
Derivative [Line Items]        
Estimated cash flows, discount rate     5.50%  
Last derivative contract expiration month and year     2018-12  
Contracts volume, derivative instruments | MMBTU     4,950,000  
Average price | $ / MMBTU 2.81 2.77 2.81 2.77
Natural gas derivative contract | 2015 through December 31, 2018        
Derivative [Line Items]        
Contracts volume, derivative instruments | MMBTU     5,220,000  
Average price | $ / MMBTU 4.41   4.41  
Cost of Sales        
Derivative [Line Items]        
Gain (loss) on derivative instruments | $ $ (510) $ (1,654) $ 87 $ (14,259)