Derivatives and Hedging-Disclosures and Fair Value Measurements - Additional Information (Detail) gal in Millions, $ in Millions	3 Months Ended
	Dec. 31, 2020 USD ($) gal	Dec. 31, 2019 gal	Sep. 30, 2020 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Hedging positions and payable amounts secured under credit facility | $	$ 1.8		$ 11.1
Prepaid expense and other current assets
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Aggregated cash posted as collateral in normal course of business | $	2.5
Interest rate swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Value | $	$ 62.8		64.0
Percentage of market risk exposure of long term debt	52.00%
Fair Value | $	$ (2.8)		$ (3.1)
Call Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	8.5	7.4
Call Option | Synthetic calls
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	86.5	85.8
Put Option
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	5.1	7.2
Swap Contracts Bought
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	11.2	17.2
Swap Contracts Bought | Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	0.6	0.2
Future Contracts | Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	29.1	42.1
Future Contracts | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative activity volume	3.8	18.2
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	5.1	4.0