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		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Interest Rate Swaps
Derivative Instruments Gain Loss [Line Items]
Income (loss) recognized in Other comprehensive income (loss)	$ 0.2	$ (8.1)		$ (7.9)
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	(0.7)	(3.8)	$ (5.2)	(6.8)
Fuel Swap Contracts
Derivative Instruments Gain Loss [Line Items]
Income (loss) recognized in Other comprehensive income (loss)	1.9	(5.5)	1.8	(5.5)
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	$ 0.4	$ (0.1)	$ (0.4)	$ 0.2