Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) gal in Millions	6 Months Ended
	Mar. 31, 2021 USD ($) gal	Mar. 31, 2020 gal	Sep. 30, 2020 USD ($)
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 | $	1,800,000
Interest rate swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Value | $	$ 61,500,000		64,000,000.0
Percentage of market risk exposure of long term debt	53.00%
Fair Value | $	$ (2,100,000)		$ (3,100,000)
Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	4.5	5.3
Call Option | Synthetic calls
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	54.4	52.8
Put Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	2.8	4.0
Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	6.4	8.9
Swap Contracts Bought | Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume		0.2
Future Contracts | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	3.8	32.6
Future Contracts | Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	25.5	52.1
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	0.5	7.3