Long-Term Debt and Bank Facility Borrowings - Additional Information (Detail) - USD ($)		Jul. 02, 2018	Jul. 30, 2015	Jun. 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	[1]			110,300,000	76,300,000
Revolving credit facility outstanding				7,800,000	0
Availability in compliance with the fixed charge coverage ratio				$ 180,600,000	166,100,000
Third 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			$ 100,000,000
Revolving credit facility expiry			Jul. 30, 2020
Additional revolving credit			$ 100,000,000
Facility size that can be increased without consulting bank group			$ 100,000,000
Term loan annual payment percentage			25.00%
Debt instrument, effective interest rate				4.80%
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%
Third 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%
Third Amendment | Quarterly
Debt Instrument [Line Items]
Term loan periodic payment			$ 2,500,000
Third Amendment | Annually | Maximum
Debt Instrument [Line Items]
Term loan periodic payment			15,000,000
Fourth 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
Revolving credit facility expiry		Jul. 02, 2023
Additional revolving credit		$ 200,000,000
Term Loan
Debt Instrument [Line Items]
Long-term debt, fair value	[1],[2]			$ 68,800,000	76,300,000
Term Loan | Third Amendment
Debt Instrument [Line Items]
Outstanding senior notes			$ 100,000,000
Senior secured term loan maturity period			5 years
Term Loan | Fourth Amendment | Subsequent Event
Debt Instrument [Line Items]
Outstanding senior notes		$ 100,000,000
Senior secured term loan maturity period		5 years
Revolving Credit Facility
Debt Instrument [Line Items]
Revolving credit facility outstanding				$ 41,500,000	$ 0
Revolving Credit Facility | Third 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 $0.4 million as of June 30, 2018 and $0.6 million as of September 30, 2017.