Investments OTTI Credit Losses Recognized in Earnings (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	$ 0.5	$ 0.8	$ 0.5	$ 0.5
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows	0.0	0.0	0.0	0.3
Ending balance	0.5	0.8	0.5	0.8
Residential mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	0.0	0.3	0.0	0.0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows	0.0	0.0	0.0	0.3
Ending balance	0.0	0.3	0.0	0.3
Commercial mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	0.5	0.5	0.5	0.5
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	$ 0.5	$ 0.5	$ 0.5