Goodwill and Intangibles - Summary of Amortization Expense of Definite Lived Intangible Assets (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015	Sep. 30, 2015
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Accumulated Amortization	$ 442.8		$ 442.8		$ 399.2
Amortization of intangibles	23.4	$ 21.2	47.0	$ 41.7
Finite-Lived Intangible Assets, Net	944.7		944.7		990.0
VOBA [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	93.9		93.9
2014	93.0		93.0
2015	86.2		86.2
2016	85.1		85.1
2017	84.9		84.9
Finite-Lived Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization of intangibles	23.4	$ 21.2	47.0	$ 41.7
Finite-Lived Intangible Assets, Net	944.7		944.7		990.0
Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Accumulated Amortization	276.0		276.0		247.4
Finite-Lived Intangible Assets, Net	711.8		711.8		737.8
Trade Names [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Accumulated Amortization	81.4		81.4		73.7
Finite-Lived Intangible Assets, Net	84.0		84.0		91.7
Technology-Based Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Accumulated Amortization	85.4		85.4		78.1
Finite-Lived Intangible Assets, Net	$ 148.9		$ 148.9		$ 160.5