Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Fixed Maturity Securities
Debt Securities, Available-for-sale, Realized Gain		$ 1.3	$ 4.4	$ 2.3
Gross Losses on Sales		(39.5)	(53.1)	(28.8)
Impairement Loss	[1]	(2.5)	0.0	0.0
Change in Allowance for Credit Losses		(3.0)	(2.2)	(4.6)
Mortgage Loans and Other Invested Assets
Gross Gains on Sales		0.5	6.0	1.4
Gross Losses on Sales		0.0	(1.0)	0.0
Impairment Loss		(7.0)	(3.0)	0.0
Credit Losses		(5.9)	(0.9)	(1.0)
Gain (Loss) on Embedded Derivative		13.0	12.4	16.2
All Other Derivatives		3.8	(0.6)	2.6
Gain (Loss) on Sale of Other Investments		9.2	0.0	0.0
Foreign Currency Transactions		(4.5)	2.0	(3.8)
Net Investment Loss		$ (34.6)	$ (36.0)	$ (15.7)

[1] 1Includes write-down of securities that we intended to sell prior to recovery of the amortized cost basis.