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Income Taxes - Additional Information (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Mar. 31, 2020
Mar. 31, 2019
Mar. 31, 2021
Sep. 30, 2019
Income tax expense $ 38.3 $ 36.2 $ 59.0 $ 75.9    
Effective income tax rate (as a percent) 35.80% 21.90% 28.90% 24.30%    
Income tax expense (benefits), tax credit, discrete items $ 11.1 $ (0.2) $ 10.0 $ 7.1    
Employee share-based payments, benefit 1.2   2.7      
Effective income tax rate reconciliation, other reconciling items, amount   (1.5)   (1.5)    
Effective income tax rate reconciliation, change in enacted tax rate, amount   $ 1.3   1.3    
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 106.5   106.5     $ 97.3
Net unrecognized tax benefits, excluding interest and penalties that would affect the company's net income if recognized 21.7   21.7      
Interest and penalties     (0.7) $ 0.6    
Accruals for payment of interest and penalties 5.1   5.1      
Scenario Forecast            
Significant change in unrecognized tax benefits is reasonably possible, amount of unrecorded benefit         $ 7.3  
Europe            
Valuation allowance on foreign net deferred tax assets $ 11.4   $ 11.4