Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Fixed Maturity Securities
Gross Gains on Sales	$ 0.5	$ 0.3
Gross Losses on Sales	0.2	8.8
Credit Losses	0.0	(4.1)
Mortgage Loans and Other Invested Assets
Gross Gains on Sales	0.2	1.4
Gross Losses on Sales	0.0	0.0
Credit Losses	(0.7)	0.5
Embedded Derivative in Modified Coinsurance Arrangement	0.3	(3.4)
All Other Derivatives	(0.6)	1.8
Foreign Currency Transactions	0.6	(1.5)
Net Investment Gain (Loss)	$ 0.1	$ (13.8)