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Income Taxes - Additional Information (Details) - USD ($)
$ in Millions
 3 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2021
Sep. 30, 2020
Income tax expense $ 13.2 $ 20.7    
Effective income tax rate (as a percent) 15.90% 21.40%    
Income tax expense (benefit), tax credit, discrete items $ (6.7) $ (1.1)    
Income tax expense (benefit) related to uncertain tax reserve releases (4.2)      
Income tax expense (benefit), interest income (2.1)      
Effective income tax rate reconciliation, other reconciling items, amount   (1.5)    
Unrecognized tax benefits 66.4     $ 79.8
Net unrecognized tax benefits, excluding interest and penalties that would affect the company's net income if recognized 17.4      
Interest and penalties 0.5 $ 0.1    
Accruals for payment of interest and penalties $ 6.2      
Scenario Forecast        
Significant change in unrecognized tax benefits is reasonably possible, amount of unrecorded benefit     $ 4.1