Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Fixed Maturity Securities
Gross Gains on Sales	$ 2.8	$ 0.5	$ 3.3	$ 0.8
Gross Losses on Sales	(13.1)	(4.2)	(13.3)	(13.0)
Credit Losses	0.0	0.0	0.0	(4.1)
Other Investments
Gross Gains on Sales	6.0	0.0	6.2	1.4
Other Investments Gross Realized Losses	0.3	0.0	0.3	0.0
Credit Losses	1.1	0.4	1.8	(0.1)
Embedded Derivative in Modified Coinsurance Arrangement	5.2	(0.8)	5.5	(4.2)
All Other Derivatives	0.6	3.7	0.0	5.5
Foreign Currency Transactions	0.8	(2.9)	1.4	(4.4)
Net Investment Gain (Loss)	$ 0.9	$ (4.1)	$ 1.0	$ (17.9)