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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
 9 Months Ended
Jun. 30, 2014
gal
Jun. 30, 2013
gal
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Aggregated cash posted as collateral in normal course of business $ 0.8  
Hedging positions and payable amounts secured under credit facility $ 0.4  
Call Option
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 1,600,000 1,300,000
Put Option
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 4,000,000 3,100,000
Inventory
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 100,000 1,000,000
Swap Contracts Bought
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 6,300,000 4,300,000
Synthetic calls
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 48,700,000 47,100,000
Futures Contracts Bought
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 52,200,000 27,300,000
Futures Contracts Sold
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 60,200,000 34,000,000
Corresponding Long And Short Swap Contracts Bought
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 20,700,000  
Hedge a Majority of its Internal Fuel Usage Swap Contracts Bought
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 1,800,000 2,000,000
Swap Contracts Sold
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume   4,400,000