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Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) $ in Thousands, gal in Millions | 12 Months Ended
--- | ---
Sep. 30, 2017 USD ($) gal | Sep. 30, 2016 USD ($) gal
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative activity volume | gal | 11.3 | 7.9
Notional Value | $ 18,000 | $ 10,800
Fair Value |  | (40)
Aggregated cash posted as collateral in normal course of business | 500 |
Hedging positions and payable amounts secured under credit facility | $ 100 | $ 300
Call Option |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative activity volume | gal | 6.1 | 6.8
Notional Value | $ 13,600 | $ 13,800
Fair Value | $ 100 | $ 300
Put Option |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative activity volume | gal | 8.1 | 6.2
Notional Value | $ 8,800 | $ 6,500
Fair Value | $ 2 | $ 20
Synthetic calls | Call Option |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative activity volume | gal | 79.0 | 84.9
Notional Value | $ 134,300 | $ 133,500
Fair Value | $ 4,100 | $ 1,300
Swap Contracts Bought |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative activity volume | gal | 15.6 | 9.9
Notional Value | $ 25,500 | $ 14,300
Fair Value | $ 2,300 | $ 1,200
Swap Contracts Sold |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative activity volume | gal | 1.3 | 1.2
Notional Value | $ 2,200 | $ 1,700
Fair Value | $ 200 | $ 200
Purchased Futures Contracts |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative activity volume | gal | 5.2 | 4.4
Notional Value | $ 8,500 | $ 6,700
Fair Value |  | $ 500
Long Swap Contracts |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative activity volume | gal | 16.8 | 21.8
Notional Value | $ 28,500 | $ 32,200
Fair Value | $ (1,900) | $ (2,100)
Hedge its Internal Fuel Usage and Other Related Activities Swap Contracts Bought |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative activity volume | gal | 6.3 | 5.7
Notional Value | $ 9,900 | $ 7,700
Fair Value | $ 1,100 | $ 1,100

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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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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.1) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.(a),2) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition Fair value of the assets less the liabilities of a derivative or group of derivatives. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13495-108611 + Details
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