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Long-Term Debt and Bank Facility Borrowings - Additional Information (Detail) - USD ($)
Jul. 30, 2015
Dec. 31, 2015
Sep. 30, 2015
Debt Instrument [Line Items]      
Hedging positions and payable amounts secured under credit facility   $ 24,800,000 $ 15,300,000
Letters of credit issued and outstanding   52,400,000 54,800,000
Availability in compliance with the fixed charge coverage ratio   184,200,000 176,000,000
Revolving Credit Facility      
Debt Instrument [Line Items]      
Revolving credit facility outstanding   $ 100,000,000 $ 100,000,000
Third Amendment      
Debt Instrument [Line Items]      
Non Seasonal maximum borrowing capacity under revolving credit facility $ 300,000,000    
Maximum borrowing capacity (heating season December to April) under revolving credit facility 450,000,000    
Issuance of line of credit for working capital purposes $ 100,000,000    
Revolving credit facility expiry Jul. 30, 2020    
Additional revolving credit $ 100,000,000    
Facility size that can be increased without consulting bank group $ 100,000,000    
Term loan annual payment percentage 25.00%    
Debt instrument, effective interest rate   3.57%  
Minimum fixed charge coverage ratio 110.00%    
Availability percentage to maximum facility size 12.50%    
Third Amendment | $100 million Term Loan      
Debt Instrument [Line Items]      
Outstanding senior notes $ 100,000,000    
Senior secured term loan maturity period 5 years    
Third Amendment | December through April      
Debt Instrument [Line Items]      
Commitment fee on the unused portion of the facility 0.30%    
Third Amendment | May through November      
Debt Instrument [Line Items]      
Commitment fee on the unused portion of the facility 0.20%    
Third Amendment | Quarterly      
Debt Instrument [Line Items]      
Term loan periodic payment $ 2,500,000    
Third Amendment | Maximum | Quarters ending June or September      
Debt Instrument [Line Items]      
Senior secured leverage ratio 300.00%    
Third Amendment | Maximum | Quarters ending December or March      
Debt Instrument [Line Items]      
Senior secured leverage ratio 450.00%    
Third Amendment | Maximum | Annually      
Debt Instrument [Line Items]      
Term loan periodic payment $ 15,000,000