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Natural Gas Derivative Instruments - Additional Information (Detail)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2016
USD ($)
$ / MMBTU
Jun. 30, 2015
USD ($)
$ / MMBTU
Jun. 30, 2016
USD ($)
MMBTU
$ / MMBTU
Jun. 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     5,820,000  
Average price | $ / MMBTU 1.95 2.64 1.95 2.64
Natural gas derivative contract | 2015 through December 31, 2018        
Derivative [Line Items]        
Contracts volume, derivative instruments | MMBTU     6,120,000  
Average price | $ / MMBTU 4.44   4.44  
Cost of Sales        
Derivative [Line Items]        
Gain (loss) on derivative instruments | $ $ 824 $ (58) $ 597 $ (12,605)