Goodwill and Intangibles, net - Summary of Amortization Expense of Definite Lived Intangible Assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Finite-Lived Intangible Assets [Line Items]
Amortization of intangibles	$ 93.9	$ 87.8	$ 81.7
Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization of intangibles			$ 81.7
VOBA
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Amortization Expense, Year Two	85.7
Finite-Lived Intangible Assets, Amortization Expense, Year Two	91.9
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
Definite Lived Intangible Assets
Finite-Lived Intangible Assets [Line Items]
Amortization of intangibles	$ 93.9	$ 87.8