Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2014 gal	Dec. 31, 2013 gal	Sep. 30, 2014
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Aggregated cash posted as collateral in normal course of business	2.3
Hedging positions and payable amounts secured under credit facility	28.7		$ 14.9
Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	5,600,000	4,500,000
Put Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	7,700,000	6,900,000
Inventory
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume		1,600,000
Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	12,400,000	10,000,000
Synthetic calls
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	94,100,000	86,000,000
Futures Contracts Sold
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	55,100,000	76,500,000
Corresponding Long And Short Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume		38,600,000
Hedge a Majority of its Internal Fuel Usage Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	2,900,000	2,400,000
Futures Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	29,600,000	57,600,000
Swap Contracts Sold
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume		16,300,000
Spread Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume		3,900,000