Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Fixed Maturity Securities
Gross Gains on Sales	$ 0.0	$ 0.5
Gross Losses on Sales	15.8	0.2
Mortgage Loans and Other Invested Assets
Gross Gains on Sales	0.0	0.2
Credit Losses	(0.5)	(0.7)
Embedded Derivative in Modified Coinsurance Arrangement	6.1	0.3
All Other Derivatives	1.6	(0.6)
Foreign Currency Transactions	(1.9)	0.6
Other Realized Gain	9.3	0.0
Net Investment Gain (Loss)	$ (1.2)	$ 0.1