Goodwill and Intangibles, including DAC and VOBA - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2013	Apr. 01, 2012	Mar. 31, 2013	Apr. 01, 2012	Sep. 30, 2012
Finite-Lived Intangible Assets [Line Items]
Intangible assets amortization method			straight-line method
Amortization of intangibles	$ (49.0)	$ (58.8)	$ (135.6)	$ (115.5)
2014	78.5		78.5
2015	78.5		78.5
2016	78.5		78.5
2017	78.5		78.5
VOBA [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period			5 years 2 months 0 days
Cumulative adjustments for net unrealized investment gains	277.6		277.6		339.4
Amortization of intangibles	(16.8)	(41.0)	(75.7)	(77.5)
2014	49.7		49.7
2015	45.6		45.6
2016	41.7		41.7
2017	35.1		35.1
DAC [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period			6 years 1 month 0 days
Cumulative adjustments for net unrealized investment gains	62.3		62.3		50.7
Amortization of intangibles	(12.1)	(2.0)	(22.7)	(7.6)
Deferred sales inducements, net of shadow adjustments	13.3		13.3		9.1
2014	17.7		17.7
2015	18.6		18.6
2016	17.9		17.9
2017	$ 16.9		$ 16.9
Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period			1 year
Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period			20 years