Long-Term Debt and Bank Facility Borrowings - Additional Information (Detail) - USD ($)		Dec. 04, 2019	Jul. 02, 2018	Sep. 30, 2019		Sep. 30, 2018
Debt Instrument [Line Items]
Hedging positions and payable amounts secured under credit facility				$ 7,700,000		$ 0
Letters of credit issued and outstanding				4,600,000		7,100,000
Long-term debt, fair value				154,000,000	[1]	101,500,000
Revolving credit facility outstanding				24,000,000		1,500,000
Availability in compliance with the fixed charge coverage ratio				126,100,000		189,000,000
Total restricted net assets				$ 250,900,000		$ 299,800,000
Fourth 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			$ 25,000,000
Senior secured term loan maturity date			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%
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%
Fourth Amendment | Quarterly
Debt Instrument [Line Items]
Term loan periodic payment			$ 2,500,000
Fourth Amendment | Maximum | Annually
Debt Instrument [Line Items]
Term loan periodic payment			$ 15,000,000
Fifth Amendment | Subsequent Event
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
Term loan periodic payment increased		$ 3,250,000
Fourth and Fifth Amendment
Debt Instrument [Line Items]
Minimum fixed charge coverage ratio			110.00%
Availability percentage to maximum facility size			12.50%
Fourth and Fifth 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%
Term Loan
Debt Instrument [Line Items]
Debt instrument, effective interest rate				5.90%		5.20%
Long-term debt, fair value	[2]			$ 92,500,000	[1]	$ 100,000,000
Term Loan | Fourth Amendment
Debt Instrument [Line Items]
Outstanding term loan			$ 100,000,000
Senior secured term loan maturity period			5 years
Long-term debt, fair value				$ 92,500,000		$ 100,000,000
Term Loan | Fifth Amendment | Subsequent Event
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				4.60%		3.80%
Revolving Credit Facility | Fourth Amendment
Debt Instrument [Line Items]
Revolving credit facility outstanding				$ 61,500,000		$ 1,500,000

[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 $0.6 million as of September 30, 2019 and $0.7 million as of September 30, 2018.