Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) gal in Millions, $ in Millions	6 Months Ended
	Mar. 31, 2016 USD ($) gal	Mar. 31, 2015 gal	Sep. 30, 2015 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Aggregated cash posted as collateral in normal course of business | $	$ 3.4
Hedging positions and payable amounts secured under credit facility | $	7.9		$ 15.3
Prepaid expenses and other current assets
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Aggregated cash posted as collateral in normal course of business | $	$ 3.4
Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	3.4	4.1
Put Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	3.5	3.1
Synthetic calls | Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	58.7	54.7
Inventory
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	5.4	6.3
Hedge its Internal Fuel Usage and Other Related Activities Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	4.6	3.2
Future Contracts | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	34.0	34.1
Future Contracts | Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	53.1	54.7
Swap Contracts | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	0.7
Spread Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	2.1