Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Mar. 31, 2015 gal	Mar. 31, 2014 gal	Sep. 30, 2014
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Aggregated cash posted as collateral in normal course of business	2.1
Hedging positions and payable amounts secured under credit facility	12.2		$ 14.9
Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	4,100,000	1,100,000
Put Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	3,100,000	4,200,000
Inventory
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume		300,000
Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	6,300,000	6,300,000
Synthetic calls
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	54,700,000	47,700,000
Futures Contracts Sold
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	54,700,000	60,000,000
Corresponding Long And Short Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume		33,100,000
Hedge its Internal Fuel Usage Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	3,200,000	1,000,000
Futures Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	34,100,000	46,200,000