Long-Term Debt and Bank Facility Borrowings - Additional Information (Detail) - USD ($)			12 Months Ended
		Dec. 04, 2019	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Debt Instrument [Line Items]
Additional term loan repayments			$ 13,000,000	$ 99,000,000	$ 7,500,000
Hedging positions and payable amounts secured under credit facility			0	11,100,000
Letters of credit issued and outstanding			3,100,000	3,500,000
Long-term debt, fair value	[1]		119,118,000	123,500,000
Revolving credit facility outstanding			8,618,000	0
Availability under credit agreement			171,500,000	203,400,000
Total restricted net assets			268,200,000	$ 245,800,000
Credit Agreement
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		$ 25,000,000
Senior secured term loan maturity date		Dec. 04, 2024
Facility size that can be increased without consulting bank group		$ 200,000,000
Term loan annual payment percentage		25.00%
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%
Credit Agreement | 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%
Credit Agreement | Quarterly
Debt Instrument [Line Items]
Term loan periodic payment		$ 3,250,000
Credit Agreement | Annually | Maximum
Debt Instrument [Line Items]
Term loan periodic payment		12,000,000
Term Loan
Debt Instrument [Line Items]
Additional term loan repayments			$ 4,600,000
Debt instrument, effective interest rate			4.30%	5.20%
Long-term debt, fair value	[1],[2]		$ 110,500,000	$ 123,500,000
Term Loan | Credit Agreement
Debt Instrument [Line Items]
Outstanding term loan		$ 130,000,000
Senior secured term loan maturity period		5 years
Revolving Credit Facility
Debt Instrument [Line Items]
Debt instrument, effective interest rate			2.50%	4.00%
Revolving credit facility outstanding			$ 8,600,000	$ 0

[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 $0.5 million as of September 30, 2021 and $0.7 million as of September 30, 2020.