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Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) gal in Millions	12 Months Ended
	Sep. 30, 2019 USD ($) gal	Sep. 30, 2018 USD ($) gal	Aug. 31, 2018 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Hedging positions and payable amounts secured under credit facility	$ 7,700,000	$ 0
Interest rate swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Value	42,500,000		$ 50,000,000
Fair Value	(2,000,000)	$ 39,000
Percentage of market risk exposure of long term debt			50.00%
Prepaid expense and other current assets
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Aggregated cash posted as collateral in normal course of business	$ 3,500,000
Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	6.1	3.2
Notional Value	$ 15,600,000	$ 8,200,000
Fair Value	$ 100,000	$ 200,000
Call Option | Synthetic calls
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	81.0	85.4
Notional Value	$ 165,400,000	$ 182,900,000
Fair Value	$ (7,200,000)	$ 14,000,000
Put Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	5.9	5.6
Notional Value	$ 7,700,000	$ 8,500,000
Fair Value	$ 28,000	$ 2,000
Swap Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	15.5	11.7
Notional Value	$ 29,700,000	$ 24,600,000
Fair Value	$ (1,000,000)	$ 2,900,000
Swap Contracts | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	1.3	1.2
Notional Value	$ 2,400,000	$ 2,600,000
Fair Value	$ (100,000)	$ 200,000
Future Contracts | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	45.2	53.1
Notional Value	$ 83,200,000	$ 114,300,000
Fair Value	$ (900,000)	$ 9,700,000
Future Contracts | Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	67.5	68.9
Notional Value	$ 124,900,000	$ 148,800,000
Fair Value	$ 200,000	$ (12,600,000)
Hedge its Internal Fuel Usage and Other Related Activities Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal	4.2	6.5
Notional Value	$ 7,400,000	$ 13,700,000
Fair Value	$ (300,000)	$ 1,000,000
Hedge its Internal Fuel Usage and Other Related Activities Swap Contracts Bought | Call Option | Synthetic calls
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume | gal		0.5
Notional Value		$ 1,000,000
Fair Value		$ 100,000

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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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+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
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