XML 72 R55.htm IDEA: XBRL DOCUMENT v3.6.0.2
Natural Gas Derivative Instruments - Additional Information (Detail)
$ in Thousands
12 Months Ended
Dec. 31, 2016
USD ($)
MMBTU
$ / MMBTU
Dec. 31, 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 4,080,000  
Average price | $ / MMBTU 2.46 2.66
Natural gas derivative contract | 2015 through December 31, 2018    
Derivative [Line Items]    
Contracts volume, derivative instruments | MMBTU 4,320,000  
Average price | $ / MMBTU 4.36  
Cost of Sales    
Derivative [Line Items]    
Gain (loss) on derivative instruments | $ $ 1,886 $ (15,040)