Schedule V Valuation and Qualifying Accounts (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	$ 29.9	$ 20.1	$ 23.5
Charged to Cost and Expense	(1.1)	(1.5)	(6.2)
Capital Loss Carryforward	0.0	0.0	0.0
Net Operating Loss Carryforward		11.3	2.7
Charged to Other Accounts	0.0	0.0	0.1
Deductions	0.0	0.0	0.0
Balance at End of Year	$ 28.8	$ 29.9	$ 20.1