Investments Investment Income (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Net Investment Income
Investment Income, Interest and Dividend		$ 2,206.9	$ 2,170.6	$ 2,173.6
Less Investment Expenses		65.2	61.8	39.4
Less Investment Income on PFA Assets		11.7	12.1	12.0
Net Investment Income		2,130.0	2,096.7	2,122.2
Fixed Maturity Securities
Net Investment Income
Investment Income, Interest and Dividend		1,860.4	1,853.7	1,849.8
Derivatives
Net Investment Income
Investment Income, Interest and Dividend		30.7	45.1	57.8
Mortgage Loans
Net Investment Income
Investment Income, Interest and Dividend		88.6	92.5	101.5
Policy Loans
Net Investment Income
Investment Income, Interest and Dividend		20.9	20.8	20.0
Perpetual Preferred Securities
Net Investment Income
Investment Income, Interest and Dividend		0.3	2.5	5.0
Private Equity Partnerships
Net Investment Income
Investment Income, Interest and Dividend	[1]	103.1	78.1	110.1
Other Long-term Investments
Net Investment Income
Investment Income, Interest and Dividend		12.0	9.2	9.4
Short-term Investments
Net Investment Income
Investment Income, Interest and Dividend		$ 90.9	$ 68.7	$ 20.0

[1] The net unrealized gain recognized in net investment income for the year ended December 31, 2024 related to private equity partnerships still held at December 31, 2024 was $127.1 million, reduced by net management fees and partnership expenses of $(24.0) million. For the years ended December 31, 2023 and 2022, the net unrealized gain recognized in net investment income related to private equity partnerships still held at year-end was $102.9 million and $124.1 million, respectively, reduced by net management fees and partnership expenses of $(24.8) million and $(14.0) million, respectively. See Note 2 for further discussion of private equity partnerships.