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, 2020	Sep. 30, 2019	Sep. 30, 2018
Interest Rate Swaps
Derivative Instruments Gain Loss [Line Items]
(Loss) income recognized in Other comprehensive income (loss)	$ (10.3)	$ (10.2)	$ 9.8
Interest Rate Swaps | Interest Expense
Derivative Instruments Gain Loss [Line Items]
Net (loss) gain reclassified from Accumulated other comprehensive loss into Interest expense/Cost of services provided	(16.9)	(8.1)	(9.8)
Fuel Swap Contracts
Derivative Instruments Gain Loss [Line Items]
(Loss) income recognized in Other comprehensive income (loss)	(2.7)	1.4	$ 0.1
Fuel Swap Contracts | Cost of Services Provided
Derivative Instruments Gain Loss [Line Items]
Net (loss) gain reclassified from Accumulated other comprehensive loss into Interest expense/Cost of services provided	$ (2.4)	$ 1.1