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Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) gal in Millions, $ in Millions	6 Months Ended
	Mar. 31, 2020 USD ($) gal	Mar. 31, 2019 gal	Sep. 30, 2019 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Hedging positions and payable amounts secured under credit facility | $	$ 18.6		$ 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 | $	1.8
Interest rate swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Value | $	$ 66.5		45.0
Percentage of market risk exposure of long term debt	51.00%
Fair Value | $	$ (3.3)		$ (2.0)
Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	5.3	2.2
Call Option | Synthetic calls
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	52.8	54.9
Put Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	4.0	3.4
Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	8.9	7.2
Swap Contracts Bought | Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume		0.8
Future Contracts | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	32.6	33.1
Future Contracts | Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	52.1	50.8
Hedge its Internal Fuel Usage and Other Related Activities Short Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	0.2
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	7.3	1.8

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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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Fair value of the assets less the liabilities of a derivative or group of derivatives.

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