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v3.21.1

Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) gal in Millions | 6 Months Ended |
--- | --- | ---
Mar. 31, 2021 USD ($) gal | Mar. 31, 2020 gal | Sep. 30, 2020 USD ($)
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 | $ | 1,800,000 |  |
Interest rate swap |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Notional Value | $ | $ 61,500,000 |  | 64,000,000.0
Percentage of market risk exposure of long term debt | 53.00% |  |
Fair Value | $ | $ (2,100,000) |  | $ (3,100,000)
Call Option |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume | 4.5 | 5.3 |
Call Option | Synthetic calls |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume | 54.4 | 52.8 |
Put Option |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume | 2.8 | 4.0 |
Swap Contracts Bought |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume | 6.4 | 8.9 |
Swap Contracts Bought | Short |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume |  | 0.2 |
Future Contracts | Long |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume | 3.8 | 32.6 |
Future Contracts | Short |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume | 25.5 | 52.1 |
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 | 0.5 | 7.3 |

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