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Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 9 Months Ended
Jun. 30, 2013
gal
Jun. 30, 2012
gal
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Aggregated cash posted as collateral in normal course of business $ 0.9  
Hedging positions secured under credit facility $ 8.0  
Call Option
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 1,300,000 1,400,000
Put Option
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 3,100,000 4,100,000
Inventory
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 1,000,000 1,200,000
Swap Contracts
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 4,300,000 4,800,000
Synthetic calls
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 47,100,000 44,600,000
Futures Contracts Bought
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 27,300,000 47,000,000
Futures Contracts Sold
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 34,000,000 51,500,000
Swap Contracts Sold
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 4,400,000  
Hedge a Majority of its Internal Fuel Usage
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 2,000,000 2,600,000