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	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Interest Rate Swaps
Derivative Instruments Gain Loss [Line Items]
Income (loss) recognized in Other comprehensive income (loss)	$ (0.2)	$ (2.2)	$ (0.3)	$ (10.2)
Interest Rate Swaps | Interest Expense
Derivative Instruments Gain Loss [Line Items]
Net (loss) income reclassified from Accumulated other comprehensive Income (loss) into Interest expense/Cost of services provided	(0.8)	(4.9)	(6.1)	(11.6)
Fuel Swap Contracts
Derivative Instruments Gain Loss [Line Items]
Income (loss) recognized in Other comprehensive income (loss)	1.2	1.5	3.2	(3.9)
Fuel Swap Contracts | Cost of Services Provided
Derivative Instruments Gain Loss [Line Items]
Net (loss) income reclassified from Accumulated other comprehensive Income (loss) into Interest expense/Cost of services provided	$ 1.2	$ (1.7)	$ 0.8	$ (1.5)