Investments Investment Income (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2024	Mar. 31, 2023
Net Investment Income
Gross Investment Income, Operating		$ 533.1	$ 526.5
Less Investment Expenses		16.6	14.7
Less Investment Income on PFA Assets		3.0	3.0
Net Investment Income		513.5	508.8
Fixed Maturity Securities
Net Investment Income
Gross Investment Income, Operating		453.9	455.1
Derivatives
Net Investment Income
Gross Investment Income, Operating		8.2	11.9
Mortgage Loans
Net Investment Income
Gross Investment Income, Operating		22.5	23.4
Policy Loans
Net Investment Income
Gross Investment Income, Operating		5.4	5.0
Perpetual Preferred Securities
Net Investment Income
Gross Investment Income, Operating	[1]	0.2	0.9
Private Equity Partnerships
Net Investment Income
Gross Investment Income, Operating	[2]	20.3	14.3
Other Long-term Investments
Net Investment Income
Gross Investment Income, Operating		2.7	1.8
Short-term Investments
Net Investment Income
Gross Investment Income, Operating		$ 19.9	$ 14.1

[1] The net unrealized gain (loss) recognized in net investment income for the three months ended March 31, 2024 related to perpetual preferred securities still held at March 31, 2024 was $(0.1) million. The net unrealized gain (loss) recognized in net investment income for the three months ended March 31, 2023 related to perpetual preferred securities still held at March 31, 2023 was $0.7 million.

[2] The net unrealized gain recognized in net investment income for the three months ended March 31, 2024 related to private equity partnerships still held at March 31, 2024 was $25.0 million, reduced by net management fees and partnership expenses of $(4.7) million. The net unrealized gain (loss) recognized in net investment income for the three months ended March 31, 2023 related to private equity partnerships still held at March 31, 2023 was $17.8 million, reduced by net management fees and partnership expense of $(3.5) million. See Note 3 for further discussion of private equity partnerships.