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Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) gal in Millions | 9 Months Ended |
--- | --- | ---
Jun. 30, 2018 USD ($) gal | Jun. 30, 2017 gal | Sep. 30, 2017 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume |  | 0.4 |
Hedging positions and payable amounts secured under credit facility | $ | $ 0 |  | $ 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 | $ | $ 400,000 |  |
Call Option |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume | 3.2 | 3.6 |
Call Option | Synthetic calls |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume | 60.4 | 55.0 |
Put Option |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume | 3.8 | 5.0 |
Swap Contracts Bought |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume | 8.4 | 9.0 |
Future Contracts | Long |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume | 53.6 | 53.3 |
Future Contracts | Short |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume | 64.2 | 67.7 |
Long Swap Contracts |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume | 0.4 | 1.3 |
Hedge its Internal Fuel Usage and Other Related Activities Swap Contracts Bought |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume | 3.1 | 6.5 |

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