Share-Based Compensation (Total Before-Tax Share-Based Compensation Cost by Type of Program) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Before-tax share-based compensation cost	$ 55.8	$ 41.2	$ 38.8
Deferred Stock Units
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Before-tax share-based compensation cost	55.8	41.1	38.3
Stock Options
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Before-tax share-based compensation cost	0.0	0.0	0.2
Restricted Stock
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Before-tax share-based compensation cost	$ 0.0	$ 0.1	$ 0.3