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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
 3 Months Ended
Dec. 31, 2013
gal
Dec. 31, 2012
gal
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Aggregated cash posted as collateral in normal course of business $ 1.6  
Hedging positions secured under credit facility $ 5.1  
Call Option
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 4,500,000 3,800,000
Put Option
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 6,900,000 7,800,000
Inventory
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 1,600,000 2,400,000
Swap Contracts Bought
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 10,000,000 10,200,000
Synthetic calls
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 86,000,000 84,800,000
Futures Contracts Bought
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 57,600,000 66,300,000
Futures Contracts Sold
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 76,500,000 76,800,000
Swap Contract Sold
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 16,300,000 30,800,000
Hedge a Majority of its Internal Fuel Usage Swap Contracts Bought
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 2,400,000 2,100,000
Swap Contracts Bought
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume   13,000,000
Corresponding Long And Short Swap Contracts Bought
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 38,600,000  
Spread Contracts Bought
    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Derivative activity volume 3,900,000