OTTI Credit Losses Recognized in Earnings (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Beginning balance	$ 27.7		$ 35.8
Credit losses for which an OTTI was previously recognized	0		0
Credit losses for which an OTTI was not previously recognized	0		0.1
Reductions for securities sold/matured	0		(0.2)
Change in recoveries of future cash flows expected to be collected	0.5	[1]	(2.3)	[1]
Reductions for previously recognized credit impairments written-down to fair value	0	[2]	(4.3)	[2]
Ending balance	28.2		29.1
Residential mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Beginning balance	27.1		34.5
Credit losses for which an OTTI was previously recognized	0		0
Credit losses for which an OTTI was not previously recognized	0		0.1
Reductions for securities sold/matured	0		0
Change in recoveries of future cash flows expected to be collected	0.6	[1]	(2.3)	[1]
Reductions for previously recognized credit impairments written-down to fair value	0	[2]	(4.0)	[2]
Ending balance	27.7		28.3
Commercial mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Beginning balance	0.6		1.3
Credit losses for which an OTTI was previously recognized	0		0
Credit losses for which an OTTI was not previously recognized	0		0
Reductions for securities sold/matured	0		(0.2)
Change in recoveries of future cash flows expected to be collected	(0.1)	[1]	0	[1]
Reductions for previously recognized credit impairments written-down to fair value	0	[2]	(0.3)	[2]
Ending balance	$ 0.5		$ 0.8

[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).