Fair Value Measurements and Derivatives 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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Interest Rate Swaps
Derivative Instruments Gain Loss [Line Items]
Income (loss) recognized in Other comprehensive income (loss)	$ (2.2)	$ (3.7)	$ (10.2)	$ (9.8)
Interest Rate Swaps | Interest Expense
Derivative Instruments Gain Loss [Line Items]
Net (loss) income reclassified from Accumulated other comprehensive income (loss) into Interest expense	(4.9)	(2.2)	(11.6)	(5.6)
Fuel Swap Contracts
Derivative Instruments Gain Loss [Line Items]
Income (loss) recognized in Other comprehensive income (loss)	1.5	0.6	(3.9)	1.5
Fuel Swap Contracts | Interest Expense
Derivative Instruments Gain Loss [Line Items]
Net (loss) income reclassified from Accumulated other comprehensive income (loss) into Interest expense	$ (1.7)	$ 0.7	$ (1.5)	$ 0.7