Stock-Based Compensation - Summary of Stock-Based Compensation Expenses (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expenses	$ 202.3	$ 125.0	$ 113.2
Employee stock investment plan [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expenses	6.7	5.2	5.8
Phantom Share Units (PSUs)
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expenses	30.4	2.7	1.7
Stock and stock unit awards [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expenses	$ 165.2	$ 117.1	$ 105.7