Goodwill and Intangibles, including DAC and VOBA - Summary of Amortization Expense of Definite Lived Intangible Assets (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Finite-Lived Intangible Assets [Line Items]
Amortization of intangibles	$ 18.2	$ 37.4	$ 50.0	$ 80.8
Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization of intangibles	12.4	11.6	24.1	23.3
Trade Names [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization of intangibles	4.1	4.1	8.2	8.2
Technology assets [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization of intangibles	4.7	4.8	9.4	9.2
Definite Lived Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization of intangibles	$ 21.2	$ 20.5	$ 41.7	$ 40.7