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Income Taxes (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2019
Mar. 31, 2018
Sep. 30, 2018
Income Tax Disclosure [Abstract]            
Income tax expense $ 108.4 $ 94.0   $ 197.4 $ 296.4  
Effective tax rate (percent) 23.40% 21.10%   23.50% 35.50%  
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent     35.00%     24.50%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent       21.00%    
Deferred tax assets net of DTL $ 188.9     $ 188.9   $ 211.7
Valuation allowance for deferred income taxes $ 17.0     $ 17.0   $ 17.7