Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2013 gal	Sep. 30, 2012 gal
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Aggregated cash posted as collateral in normal course of business	$ 1.1
Hedging positions secured under credit facility	10.5
Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	2,900,000	2,800,000
Notional Value	10.6	10.0
Fair Value	0.02	0.10
Put Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	5,000,000	6,800,000
Notional Value	11.8	16.1
Fair Value	0.04	0.10
Inventory
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	2,400,000	3,300,000
Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	6,000,000	7,100,000
Notional Value	18.0	21.1
Fair Value	(0.20)	0.80
Synthetic calls
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	81,200,000	75,200,000
Notional Value	252.8	237.5
Fair Value	(15.90)	3.70
Futures Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	17,400,000	22,600,000
Notional Value	51.9	67.6
Fair Value	(0.60)	1.70
Futures Contracts Sold
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	26,400,000	26,700,000
Notional Value	78.9	80.3
Fair Value	1.20	(1.90)
Swap Contract Sold
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	8,500,000	24,300,000
Notional Value	24.9	75.3
Fair Value	(0.30)	(0.30)
Hedge a Majority of its Internal Fuel Usage Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	3,200,000	2,200,000
Notional Value	9.0	5.7
Fair Value	0.05	0.60
Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume		19,200,000
Notional Value		60.6
Fair Value		(0.40)
Corresponding Long And Short Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	28,200,000
Notional Value	83.8
Fair Value	0.70
Spread Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	6,000,000
Notional Value	(0.5)
Fair Value	$ 0.10