Investments Investment Income (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Net Investment Income
Investment Income, Interest and Dividend		$ 2,170.6	$ 2,173.6	$ 2,260.6
Less Investment Expenses		61.8	39.4	35.1
Less Investment Income on PFA Assets		12.1	12.0	12.3
Net Investment Income		2,096.7	2,122.2	2,213.2
Fixed Maturity Securities
Net Investment Income
Investment Income, Interest and Dividend		1,853.7	1,849.8	1,888.2
Derivatives
Net Investment Income
Investment Income, Interest and Dividend		45.1	57.8	68.6
Mortgage Loans
Net Investment Income
Investment Income, Interest and Dividend		92.5	101.5	105.0
Policy Loans
Net Investment Income
Investment Income, Interest and Dividend		20.8	20.0	19.7
Perpetual Preferred Securities
Net Investment Income
Investment Income, Interest and Dividend	[1]	2.5	5.0	6.9
Private Equity Partnerships
Net Investment Income
Investment Income, Interest and Dividend	[2]	78.1	110.1	165.4
Other Long-term Investments
Net Investment Income
Investment Income, Interest and Dividend		9.2	9.4	5.5
Short-term Investments
Net Investment Income
Investment Income, Interest and Dividend		$ 68.7	$ 20.0	$ 1.3

[1] The net unrealized gain (loss) recognized in net investment income for the year ended December 31, 2023 related to perpetual preferred securities still held at December 31, 2023 was nominal. The net unrealized gain (loss) recognized in net investment income for the years ended December 31, 2022 and 2021 related to perpetual preferred securities still held at year-end was $2.8 million and $4.4 million, respectively.

[2] The net unrealized gain recognized in net investment income for the year ended December 31, 2023 related to private equity partnerships still held at December 31, 2023 was $102.9 million, reduced by net management fees and partnership expenses of $(24.8) million. For the years ended December 31, 2022 and 2021, the net unrealized gain recognized in net investment income related to private equity partnerships still held at year-end was $124.1 million and $177.7 million, respectively, reduced by net management fees and partnership expenses of $(14.0) million and $(12.3) million, respectively. See Note 2 for further discussion of private equity partnerships.