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Natural Gas Derivative Instruments - Additional Information (Detail)
$ in Thousands
3 Months Ended
Mar. 31, 2016
USD ($)
MMBTU
$ / MMBTU
Mar. 31, 2015
$ / 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 5,850,000  
Average price | $ / MMBTU 2.09 2.98
Natural gas derivative contract | 2015 through December 31, 2018    
Derivative [Line Items]    
Contracts volume, derivative instruments | MMBTU 7,020,000  
Average price | $ / MMBTU 4.46  
Cost of Sales    
Derivative [Line Items]    
Loss on derivative instruments | $ $ 227