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Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) gal in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2018 USD ($) gal | Dec. 31, 2017 gal | Sep. 30, 2018 USD ($) | Aug. 31, 2018 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |  |
Hedging positions and payable amounts secured under credit facility | $ | $ 20,200,000 |  | $ 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 | $ | 3,000,000 |  |  |
Interest rate swap |  |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |  |
Notional Value | $ |  |  |  | $ 50,000,000
Percentage of market risk exposure of long term debt |  |  |  | 50.00%
Fair Value | $ | $ (700,000) |  | $ 39,000 |
Call Option |  |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |  |
Derivative activity volume | 7.9 | 7.3 |  |
Call Option | Synthetic calls |  |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |  |
Derivative activity volume | 92.4 | 83.9 |  |
Put Option |  |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |  |
Derivative activity volume | 6.4 | 8.7 |  |
Swap Contracts Bought |  |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |  |
Derivative activity volume | 14.0 | 17.9 |  |
Future Contracts | Long |  |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |  |
Derivative activity volume | 18.3 | 27.8 |  |
Future Contracts | Short |  |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |  |
Derivative activity volume | 41.2 | 55.1 |  |
Hedge its Internal Fuel Usage and Other Related Activities Short Swap Contracts Bought |  |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |  |
Derivative activity volume | 2.0 |  |  |
Hedge its Internal Fuel Usage and Other Related Activities Swap Contracts Bought |  |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |  |
Derivative activity volume | 4.3 | 2.3 |  |

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