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Employee Benefit Plans - Deferred Compensation Plans (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]      
Defined Contribution Plan, Cost $ 16.0 $ 13.3 $ 10.8
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 31.6 28.3  
Compensation expense (reduction in expense) 4.9 4.6 4.4
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 58.2 46.7  
Compensation expense (reduction in expense) $ 6.3 $ 4.0 $ (1.7)