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Derivatives and Hedging-Fair Value Measurements and Accounting for the Offsetting of Certain Contracts - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | 6 Months Ended
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Mar. 31, 2014 gal | Mar. 31, 2013 gal
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Aggregated cash posted as collateral in normal course of business | $ 1.6 |
Hedging positions and payable amounts secured under credit facility | $ 7.1 |
Call Option |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative activity volume | 1,100,000 | 900,000
Put Option |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative activity volume | 4,200,000 | 3,700,000
Inventory |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative activity volume | 300,000 | 700,000
Swap Contracts Bought |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative activity volume | 6,300,000 | 5,000,000
Synthetic calls |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative activity volume | 47,700,000 | 45,400,000
Futures Contracts Bought |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative activity volume | 46,200,000 | 48,500,000
Futures Contracts Sold |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative activity volume | 60,000,000 | 54,300,000
Corresponding Long And Short Swap Contracts Bought |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative activity volume | 33,100,000 |
Hedge a Majority of its Internal Fuel Usage Swap Contracts Bought |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative activity volume | 1,000,000 | 800,000
Swap Contracts Sold |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative activity volume |  | 3,000,000

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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 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 2 -Article 7 + Details
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Balance Type: | debit
Period Type: | instant

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- Definition Amount of the hedged item as of the balance sheet date related to the derivative. For example, the hedged balance on a debt instrument. + References No definition available. + Details
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- Details
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- Definition Nominal volume used to calculate payments on a derivative instrument. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
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