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Goodwill and Intangibles, net - Additional Information (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2018
Mar. 31, 2017
Sep. 30, 2017
Finite-Lived Intangible Assets [Line Items]          
Indefinite-Lived Intangible Assets (Excluding Goodwill) $ 1,029.0   $ 1,029.0   $ 1,024.3
Document Period End Date     Mar. 31, 2018    
Finite-Lived Intangible Assets, Accumulated Amortization 314.8   $ 314.8   $ 297.1
Finite-Lived Intangible Assets, Amortization Expense, Year Two 57.5   57.5    
Definite Lived Intangible Assets          
Finite-Lived Intangible Assets [Line Items]          
Amortization of intangibles 14.4 $ 15.1 29.4 $ 30.5  
VOBA          
Finite-Lived Intangible Assets [Line Items]          
Finite-Lived Intangible Assets, Amortization Expense, Year Two 57.4   57.4    
Finite-Lived Intangible Assets, Amortization Expense, Year Three 55.0   55.0    
Finite-Lived Intangible Assets, Amortization Expense, Year Four 49.7   49.7    
Finite-Lived Intangible Assets, Amortization Expense, Year Five $ 48.0   $ 48.0