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Income Taxes - Additional Information (Details) - USD ($)
$ in Millions
 3 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2020
Sep. 30, 2019
Income tax expense $ 20.7 $ 39.7    
Effective income tax rate (as a percent) 21.40% 26.90%    
Income tax expense (benefit), tax credit, discrete items $ (1.1) $ 7.3    
Employee share-based payments, benefit (1.5)      
Income tax expense (benefit) related to uncertain tax position reserve   6.2    
Tax cuts and jobs act, incomplete accounting, transition tax for accumulated foreign earnings, provisional income tax expense   0.8    
Unrecognized tax benefits 106.0     $ 97.3
Net unrecognized tax benefits, excluding interest and penalties that would affect the company's net income if recognized 21.4      
Interest and penalties (0.1) $ 0.2    
Accruals for payment of interest and penalties $ 4.3      
Scenario Forecast        
Significant change in unrecognized tax benefits is reasonably possible, amount of unrecorded benefit     $ 7.2