Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) (USD $)	12 Months Ended
	Sep. 30, 2014 gal	Sep. 30, 2013 gal
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Aggregated cash posted as collateral in normal course of business	$ 2,500,000
Hedging positions and payable amounts secured under credit facility	14,900,000	10,500,000
Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	3,100,000	2,900,000
Notional Value	10,800,000	10,600,000
Fair Value	10,000	20,000
Put Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	8,500,000	5,000,000
Notional Value	19,300,000	11,800,000
Fair Value	100,000	40,000
Inventory
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	13,900,000	2,400,000
Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume		6,000,000
Notional Value	39,900,000	18,000,000
Fair Value	(2,900,000)	(200,000)
Synthetic calls
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	83,800,000	81,200,000
Notional Value	252,300,000	252,800,000
Fair Value	(28,800,000)	(15,900,000)
Futures Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume		17,400,000
Notional Value		51,900,000
Fair Value		(600,000)
Futures Contracts Sold
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume		26,400,000
Notional Value		78,900,000
Fair Value		1,200,000
Swap Contracts Sold
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	18,700,000	8,500,000
Notional Value	52,000,000	24,900,000
Fair Value	2,300,000	(300,000)
Corresponding Long And Short Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	12,400,000	28,200,000
Notional Value	36,200,000	83,800,000
Fair Value	(200,000)	700,000
Hedge a Majority of its Internal Fuel Usage Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	4,300,000	3,200,000
Notional Value	12,000,000	9,000,000
Fair Value	(900,000)	50,000
Spread Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume		6,000,000
Notional Value		(500,000)
Fair Value		$ 100,000