Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) gal in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2020 USD ($) gal	Sep. 30, 2019 USD ($) gal
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Hedging positions and payable amounts secured under credit facility	$ 11.1	$ 7.7
Prepaid expense and other current assets
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Aggregated cash posted as collateral in normal course of business	0.1
Interest rate swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Value	64.0	45.0
Fair Value	$ (3.1)	$ (2.0)
Percentage of market risk exposure of long term debt	52.00%
Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	8.5	6.1
Notional Value	$ 17.4	$ 15.6
Fair Value	$ 0.1	$ 0.1
Call Option | Synthetic calls
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	80.7	81.0
Notional Value	$ 123.4	$ 165.4
Fair Value	$ (8.6)	$ (7.2)
Put Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	5.1	5.9
Notional Value	$ 4.2	$ 7.7
Fair Value	$ 0.2	$ 28.0
Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	11.3	15.5
Notional Value	$ 16.0	$ 29.7
Fair Value	$ (2.6)	$ (1.0)
Swap Contracts Bought | Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	19.2
Notional Value	$ 22.7
Swap Contracts Bought | Short | Maximum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair Value	$ 0.1
Swap Contracts Bought | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal		1.3
Notional Value		$ 2.4
Fair Value		$ (0.1)
Future Contracts | Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	11.1	67.5
Notional Value	$ 14.3	$ 124.9
Fair Value	$ 0.7	$ 0.2
Future Contracts | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	3.8	45.2
Notional Value	$ 5.4	$ 83.2
Fair Value	$ (0.4)	$ (0.9)
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	7.0	4.2
Notional Value	$ 10.7	$ 7.4
Fair Value	$ (0.4)	$ (0.3)