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Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) gal in Millions | 12 Months Ended |
--- | --- | ---
Sep. 30, 2018 USD ($) gal | Sep. 30, 2017 USD ($) gal | Aug. 31, 2018 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Hedging positions and payable amounts secured under credit facility | $ 0 | $ 100,000 |
Interest rate swap |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Notional Value |  |  | $ 50,000,000
Fair Value | 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 | $ 1,200,000 |  |
Call Option |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume | gal | 3.2 | 6.1 |
Notional Value | $ 8,200,000 | $ 13,600,000 |
Fair Value | $ 200,000 | $ 100,000 |
Call Option | Synthetic calls |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume | gal | 85.4 | 79.0 |
Notional Value | $ 182,900,000 | $ 134,300,000 |
Fair Value | $ 14,000,000 | $ 4,100,000 |
Put Option |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume | gal | 5.6 | 8.1 |
Notional Value | $ 8,500,000 | $ 8,800,000 |
Fair Value | $ 2,000 | $ 2,000 |
Swap Contracts Bought |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume | gal | 11.7 | 15.6 |
Notional Value | $ 24,600,000 | $ 25,500,000 |
Fair Value | $ 2,900,000 | $ 2,300,000 |
Swap Contracts Sold |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume | gal | 1.2 | 1.3 |
Notional Value | $ 2,600,000 | $ 2,200,000 |
Fair Value | $ 200,000 | $ 200,000 |
Long Swap Contracts |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume | gal |  | 16.8 |
Notional Value |  | $ 28,500,000 |
Fair Value |  | $ (1,900,000) |
Purchased Futures Contracts |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume | gal | 53.1 | 5.2 |
Notional Value | $ 114,300,000 | $ 8,500,000 |
Fair Value | $ 9,700,000 |  |
Sold Futures Contracts |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative activity volume | gal | 68.9 | 11.3 |
Notional Value | $ 148,800,000 | $ 18,000,000 |
Fair Value | $ (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 | 6.5 | 6.3 |
Notional Value | $ 13,700,000 | $ 9,900,000 |
Fair Value | $ 1,000,000 | $ 1,100,000 |
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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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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