Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) gal in Millions	9 Months Ended
	Jun. 30, 2021 USD ($) gal	Jun. 30, 2020 gal	Sep. 30, 2020 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Hedging positions and payable amounts secured under credit facility | $	$ 0		$ 11,100,000
Prepaid expense and other current assets
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Aggregated cash posted as collateral in normal course of business | $	3,600,000
Interest rate swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Value | $	$ 60,300,000		64,000,000.0
Percentage of market risk exposure of long term debt	53.00%
Fair Value | $	$ (1,800,000)		$ (3,100,000)
Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	5.3	6.3
Call Option | Synthetic calls
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	57.2	59.5
Put Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	2.9	3.5
Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	8.9	8.2
Swap Contracts Bought | Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume		19.4
Future Contracts | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	3.8	57.2
Future Contracts | Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	21.5	59.6
Hedge its Internal Fuel Usage and Other Related Activities Call Options and Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	3.6	7.9