Goodwill and Intangibles, net - Estimated Amortization Expense in Future Fiscal Periods (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Schedule Of Amortization Expense And Weighted Average Lives Of Identified Intangible Assets [Line Items]
Amortization of intangibles	$ 93.9	$ 87.8	$ 81.7
Finite-Lived Intangible Assets, Accumulated Amortization	488.7	$ 399.2
VOBA
Schedule Of Amortization Expense And Weighted Average Lives Of Identified Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Amortization Expense, Year Two	91.9
Finite-Lived Intangible Assets, Amortization Expense, Year Two	85.7
Finite-Lived Intangible Assets, Amortization Expense, Year Three	85.4
Finite-Lived Intangible Assets, Amortization Expense, Year Four	85.2
Finite-Lived Intangible Assets, Amortization Expense, Year Five	$ 81.9