Goodwill and Intangibles, net - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Finite-Lived Intangible Assets [Line Items]
Goodwill, Acquired During Period			$ 1,054.7
Amortization of intangibles	$ 93.9	$ 87.8	$ 81.7
Finite-Lived Intangible Assets, Accumulated Amortization	488.7	399.2
VOBA
Finite-Lived 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
FOH [Member]
Finite-Lived Intangible Assets [Line Items]
Impaired Financing Receivable, Recorded Investment		$ 16.2