Inventories - Additional Information (Detail) - gal gal in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Inventory [Line Items]
Heating oil and other fuel inventories	26.9	22.8
Maximum product supply by other supplier		10.00%
Shell Oil Company
Inventory [Line Items]
Percentage of product purchases from supplier	13.00%
Motiva Enterprises LLC
Inventory [Line Items]
Percentage of product purchases from supplier	12.00%
Global Companies LLC
Inventory [Line Items]
Percentage of product purchases from supplier	11.00%
Home heating oil and propane
Inventory [Line Items]
Market based product supply contracts for next twelve months	240.5
Diesel and gasoline
Inventory [Line Items]
Market based product supply contracts for next twelve months	44.2