Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) gal in Millions, $ in Millions	9 Months Ended
	Jun. 30, 2017 USD ($) gal	Jun. 30, 2016 gal	Sep. 30, 2016 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Hedging positions secured under credit facility | $	$ 4.9		$ 0.3
Prepaid expenses and other current assets
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Aggregated cash posted as collateral in normal course of business | $	$ 0.5
Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	3.6	4.5
Put Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	5.0	4.1
Synthetic calls | Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	55.0	60.0
Inventory
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	9.0	6.6
Swap Contracts | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	1.3	1.1
Future Contracts | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	53.3	54.0
Future Contracts | Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	67.7	70.1
Spread Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	0.4	0.7
Hedge its Internal Fuel Usage and Other Related Activities Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	6.5	5.9