Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) gal in Millions	12 Months Ended
	Sep. 30, 2021 USD ($) gal	Sep. 30, 2020 USD ($) gal
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Hedging positions and payable amounts secured under credit facility	$ 0	$ 11,100,000
Prepaid expense and other current assets
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Aggregated cash posted as collateral in normal course of business	5,100,000
Interest rate swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Value	59,000,000.0	64,000,000.0
Fair Value	$ (1,600,000)	$ (3,100,000)
Percentage of market risk exposure of long term debt	53.00%
Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	7.8	8.5
Notional Value	$ 18,900,000	$ 17,400,000
Fair Value	$ 1,200,000	$ 100,000
Call Option | Synthetic calls
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	74.2	80.7
Notional Value	$ 143,200,000	$ 123,400,000
Fair Value	$ 23,700,000	$ (8,600,000)
Put Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	4.4	5.1
Notional Value	$ 5,800,000	$ 4,200,000
Fair Value	$ 100,000	$ 200,000
Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	11.8	11.3
Notional Value	$ 22,900,000	$ 16,000,000.0
Fair Value	$ 1,400,000	$ (2,600,000)
Swap Contracts Bought | Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal		19.2
Notional Value		$ 22,700,000
Swap Contracts Bought | Short | Maximum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair Value		$ 100,000
Future Contracts | Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	21.0	11.1
Notional Value	$ 42,100,000	$ 14,300,000
Fair Value	$ (6,800,000)	$ 700,000
Future Contracts | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	3.8	3.8
Notional Value	$ 5,400,000	$ 5,400,000
Fair Value	$ 3,400,000	$ (400,000)
Hedge its Internal Fuel Usage and Other Related Activities Call Options and Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	6.8	7.0
Notional Value	$ 13,800,000	$ 10,700,000
Fair Value	$ 1,500,000	$ (400,000)