Fair Value Measurements and Derivative Instruments - Summary of Effects on Consolidated Financial Statements of Designated As Cash Flow Hedges (Details) - Cash Flow Hedges - Designated - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Interest Rate Swaps
Derivative Instruments Gain Loss [Line Items]
(Loss) recognized in Other comprehensive income (loss)	$ (0.6)	$ (10.3)	$ (10.2)
Interest Rate Swaps | Interest Expense
Derivative Instruments Gain Loss [Line Items]
Net loss reclassified from Accumulated other comprehensive loss into Interest expense	(6.8)	(16.9)	(8.1)
Fuel Swap Contracts
Derivative Instruments Gain Loss [Line Items]
(Loss) recognized in Other comprehensive income (loss)	3.5	(2.7)	1.4
Fuel Swap Contracts | Cost of Services Provided
Derivative Instruments Gain Loss [Line Items]
Net loss reclassified from Accumulated other comprehensive loss into Interest expense	$ 2.3	$ (2.4)	$ 1.1