Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Fixed Maturity Securities
Debt Securities, Available-for-sale, Realized Gain	$ 4.4	$ 2.3	$ 76.2	[1]
Gross Losses on Sales	(53.1)	(28.8)	(11.5)
Credit Losses	(2.2)	(4.6)	(9.3)
Mortgage Loans and Other Invested Assets
Gross Gains on Sales	6.0	1.4	5.8
Gross Losses on Sales	(1.0)	0.0	0.0
Impairment Loss	(3.0)	0.0	0.0
Credit Losses	(0.9)	(1.0)	4.7
Gain (Loss) on Embedded Derivative	12.4	16.2	9.7
All Other Derivatives	(0.6)	2.6	3.1
Foreign Currency Transactions	2.0	(3.8)	(2.0)
Net Investment Gain (Loss)	$ (36.0)	$ (15.7)	$ 76.7

[1] Gross gains on sales of fixed maturity securities for the year ended December 31, 2021 includes gains of $67.6 million as a result of the second phase of the reinsurance transaction that we completed during the first quarter of 2021. See Note 14 for further discussion.