Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) gal in Millions	9 Months Ended
	Jun. 30, 2018 USD ($) gal	Jun. 30, 2017 gal	Sep. 30, 2017 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume		0.4
Hedging positions and payable amounts secured under credit facility | $	$ 0		$ 100,000
Prepaid expense and other current assets
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Aggregated cash posted as collateral in normal course of business | $	$ 400,000
Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	3.2	3.6
Call Option | Synthetic calls
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	60.4	55.0
Put Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	3.8	5.0
Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	8.4	9.0
Future Contracts | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	53.6	53.3
Future Contracts | Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	64.2	67.7
Long Swap Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	0.4	1.3
Hedge its Internal Fuel Usage and Other Related Activities Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	3.1	6.5