Investments OTTI Credit Losses Recognized in Earnings (Detail) - Fixed maturities - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	$ 0.3	$ 12.2	$ 11.5	$ 12.8
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment	0.4	0.0	0.4	0.0
Reductions for securities sold/matured	0.0	0.0	(11.2)	0.0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows	0.0	(0.3)	0.0	(0.9)
Ending balance	0.7	11.9	0.7	11.9
Residential mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	0.2	11.8	11.1	12.4
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment	0.0	0.0	0.0	0.0
Reductions for securities sold/matured	0.0	0.0	(10.9)	0.0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows	0.0	(0.3)	0.0	(0.9)
Ending balance	0.2	11.5	0.2	11.5
Commercial mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	0.1	0.4	0.4	0.4
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment	0.4	0.0	0.4	0.0
Reductions for securities sold/matured	0.0	0.0	(0.3)	0.0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows	0.0	0.0	0.0	0.0
Ending balance	$ 0.5	$ 0.4	$ 0.5	$ 0.4