Long-Term Debt and Bank Facility Borrowings - Additional Information (Detail) - USD ($)		Jul. 02, 2018	Jul. 30, 2015	Dec. 31, 2018	Sep. 30, 2018
Debt Instrument [Line Items]
Hedging positions and payable amounts secured under credit facility				$ 20,200,000	$ 0
Letters of credit issued and outstanding				7,100,000	7,100,000
Long-term debt, fair value	[1]			192,500,000	101,500,000
Revolving credit facility outstanding				92,500,000	1,500,000
Availability in compliance with the fixed charge coverage ratio				180,300,000	189,000,000
Term Loan
Debt Instrument [Line Items]
Long-term debt, fair value	[1],[2]			100,000,000	100,000,000
Revolving Credit Facility
Debt Instrument [Line Items]
Revolving credit facility outstanding				$ 92,500,000	$ 1,500,000
Fourth Amendment
Debt Instrument [Line Items]
Non Seasonal maximum borrowing capacity under revolving credit facility		$ 300,000,000	$ 300,000,000
Maximum borrowing capacity (heating season December to April) under revolving credit facility		450,000,000	450,000,000
Issuance of line of credit for working capital purposes		$ 25,000,000
Revolving credit facility expiry		Jul. 02, 2023
Additional revolving credit		$ 200,000,000
Facility size that can be increased without consulting bank group		$ 200,000,000
Term loan annual payment percentage		25.00%
Debt instrument, effective interest rate				5.80%	5.20%
Commitment fee on the unused portion of the facility from December through April		0.30%
Commitment fee on the unused portion of the facility from May through November		0.20%
Minimum fixed charge coverage ratio		110.00%
Availability percentage to maximum facility size		12.50%
Fourth Amendment | Maximum
Debt Instrument [Line Items]
Senior secured leverage ratio during quarters ending June or September		300.00%
Senior secured leverage ratio during quarters ending December or March		450.00%
Fourth Amendment | Quarterly
Debt Instrument [Line Items]
Term loan periodic payment		$ 2,500,000
Fourth Amendment | Annually | Maximum
Debt Instrument [Line Items]
Term loan periodic payment		15,000,000
Fourth Amendment | Term Loan
Debt Instrument [Line Items]
Outstanding senior notes		$ 100,000,000	$ 100,000,000
Senior secured term loan maturity period		5 years	5 years
Fourth Amendment | Revolving Credit Facility
Debt Instrument [Line Items]
Debt instrument, effective interest rate				5.10%	3.80%

[1] The face amount of the Company’s variable rate long-term debt approximates fair value.

[2] Carrying amounts are net of unamortized debt issuance costs of $0.7 million as of December 31, 2018 and September 30, 2018.