Long-Term Debt and Bank Facility Borrowings - Additional Information (Detail) - USD ($)		Jul. 02, 2018	Jul. 30, 2015	Sep. 30, 2018		Sep. 30, 2017
Debt Instrument [Line Items]
Hedging positions and payable amounts secured under credit facility				$ 0		$ 100,000
Letters of credit issued and outstanding				7,100,000		48,000,000
Long-term debt, fair value				101,500,000	[1]	76,300,000
Revolving credit facility outstanding				1,500,000
Availability in compliance with the fixed charge coverage ratio				189,000,000		166,100,000
Total restricted net assets				$ 299,800,000		$ 296,200,000
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				5.20%		4.10%
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 | Quarterly
Debt Instrument [Line Items]
Term loan periodic payment		$ 2,500,000
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 | Maximum | Annually
Debt Instrument [Line Items]
Term loan periodic payment		$ 15,000,000
Term Loan
Debt Instrument [Line Items]
Long-term debt, fair value	[2]			$ 100,000,000	[1]	$ 76,300,000
Term Loan | Fourth Amendment
Debt Instrument [Line Items]
Outstanding senior notes		$ 100,000,000	$ 100,000,000
Senior secured term loan maturity period		5 years	5 years
Term Loan | Third Amendment
Debt Instrument [Line Items]
Long-term debt, fair value						76,300,000
Revolving Credit Facility
Debt Instrument [Line Items]
Revolving credit facility outstanding				$ 1,500,000		$ 0
Revolving Credit Facility | Fourth Amendment
Debt Instrument [Line Items]
Debt instrument, effective interest rate				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 September 30, 2018 and $0.6 million as of September 30, 2017.