OTTI Credit Losses Recognized in Earnings (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	$ 27.4		$ 28.8		$ 27.7		$ 35.8
Credit losses for which an OTTI was previously recognized	0		0		0		0.1
Credit losses for which an OTTI was not previously recognized	0		0		0		0.2
Reductions for securities sold/matured	0		0		0		(0.2)
Change in recoveries of future cash flows expected to be collected	(2.5)	[1]	(0.5)	[1]	(2.6)	[1]	(3.3)	[1]
Reductions for previously recognized credit impairments written-down to fair value	0	[2]	0	[2]	(0.2)	[2]	(4.3)	[2]
Ending balance	24.9		28.3		24.9		28.3
Residential mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	26.9		28.1		27.1		34.5
Credit losses for which an OTTI was previously recognized	0		0		0		0.1
Credit losses for which an OTTI was not previously recognized	0		0		0		0.2
Reductions for securities sold/matured	0		0		0		0
Change in recoveries of future cash flows expected to be collected	(2.4)	[1]	(0.4)	[1]	(2.4)	[1]	(3.1)	[1]
Reductions for previously recognized credit impairments written-down to fair value	0	[2]	0	[2]	(0.2)	[2]	(4.0)	[2]
Ending balance	24.5		27.7		24.5		27.7
Commercial mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	0.5		0.7		0.6		1.3
Credit losses for which an OTTI was previously recognized	0		0		0		0
Credit losses for which an OTTI was not previously recognized	0		0		0		0
Reductions for securities sold/matured	0		0		0		(0.2)
Change in recoveries of future cash flows expected to be collected	(0.1)	[1]	(0.1)	[1]	(0.2)	[1]	(0.2)	[1]
Reductions for previously recognized credit impairments written-down to fair value	0	[2]	0	[2]	0	[2]	(0.3)	[2]
Ending balance	$ 0.4		$ 0.6		$ 0.4		$ 0.6

[1] Reflects expected recovery of prior period impairments that will be accreted into income over the remaining life of the security, net of any current quarter decreases in expected cash flows on previously recorded reductions.

[2] Reflects reductions of prior credit impairments where the current credit impairment requires writing securities down to fair value (i.e., no remaining non-credit loss).