Goodwill and Intangibles - Summary of Amortization Expense of Definite Lived Intangible Assets (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Sep. 30, 2015
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Accumulated Amortization	$ 421.3		$ 399.2
Amortization of intangibles	23.6	$ 20.5
Finite-Lived Intangible Assets, Net	962.5		990.0
VOBA [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	93.9
2014	93.0
2015	86.2
2016	85.1
2017	84.9
Finite-Lived Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization of intangibles	23.6	$ 20.5
Finite-Lived Intangible Assets, Net	962.5		990.0
Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Accumulated Amortization	259.8		247.4
Finite-Lived Intangible Assets, Net	720.1		737.8
Trade Names [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Accumulated Amortization	77.5		73.7
Finite-Lived Intangible Assets, Net	87.9		91.7
Technology-Based Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Accumulated Amortization	84.0		78.1
Finite-Lived Intangible Assets, Net	$ 154.5		$ 160.5