Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Fixed Maturity Securities
Gross Gains on Sales	$ 1.1	$ 0.4	$ 1.1	$ 3.7
Gross Losses on Sales	(8.5)	(37.4)	(29.7)	(50.7)
Impairment Loss	(1.3)	0.0	(2.5)	0.0
credit losses	(0.2)	0.0	(3.1)	0.0
Other Investments
Gross Gains on Sales	0.4	0.0	9.7	6.0
Other Investments Gross Realized Losses	0.0	0.9	0.0	1.0
Impairment Loss	0.0	3.0	0.0	3.0
Current Period Provisions	4.5	(0.9)	6.4	0.9
Embedded Derivative in Modified Coinsurance Arrangement	(0.8)	9.2	6.1	14.7
All Other Derivatives	(2.2)	2.0	(0.2)	2.0
Foreign Currency Transactions	3.1	(2.2)	0.5	(0.8)
Net Investment Loss	$ (12.9)	$ (31.0)	$ (24.5)	$ (30.0)