Long-Term Debt and Bank Facility Borrowings - Additional Information (Detail) - USD ($)				12 Months Ended
		Sep. 30, 2025	Sep. 27, 2024	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Debt Instrument [Line Items]
Additional loan repayments				$ 21,000,000	$ 148,500,000	$ 16,500,000
Hedging positions and payable amounts secured under credit facility		$ 1,300,000		1,300,000	14,200,000
Letters of credit issued and outstanding		5,100,000		5,100,000	5,200,000
Long-term debt, fair value	[1]	189,000,000		189,000,000	210,005,000
Revolving credit facility outstanding		0		0	5,000
Availability under credit agreement		165,000,000		165,000,000	166,500,000
Total restricted net assets		286,900,000		286,900,000	$ 254,000,000
Credit Agreement
Debt Instrument [Line Items]
Non Seasonal maximum borrowing capacity under revolving credit facility			$ 400,000,000
Maximum borrowing capacity (heating season December to April) under revolving credit facility			475,000,000
Issuance of line of credit for working capital purposes			$ 25,000,000
Senior secured term loan maturity date			Sep. 27, 2029
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%
Facility size that can be increased additional without consulting bank group				$ 200,000,000
Maximum | Credit Agreement
Debt Instrument [Line Items]
Availability percentage to maximum facility size			12.50%
Senior secured leverage ratio during quarters ending June or September			300.00%
Senior secured leverage ratio during quarters ending December or March			550.00%
Maximum facility size		$ 21,400,000	$ 35,000,000
Quarterly | Credit Agreement
Debt Instrument [Line Items]
Term loan periodic payment			5,300,000
Annually | Maximum | Credit Agreement
Debt Instrument [Line Items]
Term loan periodic payment			4,000,000
Term Loan
Debt Instrument [Line Items]
Debt instrument, effective interest rate		7.10%		7.10%	7.20%
Long-term debt, fair value	[1],[2]	$ 189,000,000		$ 189,000,000	$ 210,000,000
Term Loan | Credit Agreement
Debt Instrument [Line Items]
Outstanding term loan			$ 210,000,000
Senior secured term loan maturity period			5 years
Revolving Credit Facility
Debt Instrument [Line Items]
Debt instrument, effective interest rate		6.40%		6.40%	7.50%
Revolving credit facility outstanding		$ 0		$ 0	$ 100,000

[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 $0.9 million as of September 30, 2025 and $1.2 million as of September 30, 2024.