Fair Value Measurements and Derivative Instruments - Additional Information (Details)	9 Months Ended
	Jun. 30, 2021 USD ($) Fuel gal	Sep. 30, 2020 USD ($)
Interest Rate Contracts
Derivative [Line Items]
Notional amount	$ 500,000,000.0	$ 980,000,000.0
Interest Rate Swaps
Derivative [Line Items]
Net deferred losses on interest rate swap net of taxes expected to be recognized over the next 12 months	$ 2,200,000
Fuel Swap Contracts
Derivative [Line Items]
Number of outstanding fuel contracts | Fuel	2
Notional volume | gal	5.3
Net deferred gain on fuel swap net of taxes expected to be recognized over the next 12 months	$ 1,800,000
Maximum
Derivative [Line Items]
Percentage of participants compensation deferred	70.00%