Carrying amount as of the balance sheet date of cash collateral held for borrowed securities, for which the cash is restricted as to withdrawal or usage.

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Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) gal in Millions	6 Months Ended
	Mar. 31, 2023 USD ($) gal	Mar. 31, 2022 gal	Sep. 30, 2022 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Hedging positions or payable amounts secured under credit facility \| $	$ 13,500,000		$ 0
Prepaid expense and other current assets
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Aggregated cash posted as collateral in normal course of business \| $	2,000,000.0
Interest rate swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Value \| $	$ 84,800,000		54,000,000.0
Percentage of market risk exposure of long term debt	54.00%
Fair Value \| $	$ 1,000,000.0		$ 2,000,000.0
Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	6.5	37.1
Call Option \| Synthetic calls
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	36.7	15.5
Put Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	2.4	2.2
Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	5.4	6.2
Swap Contracts Bought \| Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	13.5
Future Contracts \| Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume		2.9
Future Contracts \| Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	1.8	17.1
Hedge its Internal Fuel Usage and Other Related Activities Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	3.7
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		1.0