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
	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