Investments OTTI Credit Losses Recognized in Earnings (Detail) - Fixed maturities - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance		$ 12.6	$ 19.5	$ 13.1	$ 19.6
Change in recoveries of future cash flows expected to be collected	[1]	1.8	(6.1)	1.3	(6.2)
Ending balance		14.4	13.4	14.4	13.4
Residential mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance		12.2	19.1	12.7	19.2
Change in recoveries of future cash flows expected to be collected	[1]	1.8	(6.1)	1.3	(6.2)
Ending balance		14.0	13.0	14.0	13.0
Commercial mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance		0.4	0.4	0.4	0.4
Change in recoveries of future cash flows expected to be collected	[1]	0.0	0.0	0.0	0.0
Ending balance		$ 0.4	$ 0.4	$ 0.4	$ 0.4

[1] Reflects the current period change in the expected recovery of prior impairments that will be accreted into income over the remaining life of the security.