Revenue Recognition - Additional Information (Detail) - USD ($)	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Revenue Recognition [Line Items]
Revenue, practical expedient, incremental cost of obtaining contract [true false]	true
Contract costs, amortization period	5 years
Contract costs, impairment loss	$ 0	$ 0	$ 0
Contract liabilities	141,600,000	139,600,000
Contract with customer liability, revenue recognized	128,500,000	118,600,000
Prepaid Expense and Other Current Assets
Revenue Recognition [Line Items]
Deferred contract costs,current	3,400,000	3,400,000
Deferred Charges and Other Assets, Net
Revenue Recognition [Line Items]
Deferred contract costs,non current	5,700,000	5,900,000
Delivery and Branch Expenses
Revenue Recognition [Line Items]
Amortization of deferred contract costs	$ 3,900,000	$ 4,000,000.0
Maximum
Revenue Recognition [Line Items]
Contract liabilities recognition service contract period	1 year
Equipment Maintenance Service Contracts | Maximum
Revenue Recognition [Line Items]
Revenue from service contracts period of recognition	1 year
Residential and Commercial Home Heating Oil Customers
Revenue Recognition [Line Items]
Percentage of customers receiving deliveries based on prevailing weather conditions	95.00%
Residential Customers
Revenue Recognition [Line Items]
Customer payment terms	30 days
Percentage of customers take advantage of smart pay budget payment plan	33.00%