Employee Benefit Plans - Deferred Compensation Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Defined Contribution Plan, Cost	$ 18.4	$ 16.0	$ 13.3
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	35.4	31.6
Compensation expense (reduction in expense)	5.4	4.9	4.6
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	69.3	58.2
Compensation expense (reduction in expense)	$ 5.8	$ 6.3	$ 4.0