Employee Benefit Plans - Deferred Compensation Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Defined Contribution Plan, Cost	$ 23.4	$ 21.7	$ 18.4
Supplemental Executive Retirement Plan [Member]
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Deferred Compensation Liability, Current and Noncurrent	44.5	40.6
Compensation expense (reduction in expense)	6.5	5.8	5.4
Deferred Compensation Plan For Select Group Of Employees [Member]
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Deferred Compensation Liability, Current and Noncurrent	91.0	78.6
Compensation expense (reduction in expense)	$ 5.3	$ 2.9	$ 5.8