Investments Investment Income (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2026	Mar. 31, 2025
Net Investment Income
Gross Investment Income, Operating		$ 508.1	$ 537.5
Less Investment Expenses		21.8	21.4
Less Investment Income on PFA Assets		2.9	2.9
Net Investment Income		483.4	513.2
Fixed Maturity Securities
Net Investment Income
Gross Investment Income, Operating		425.4	463.3
Derivatives
Net Investment Income
Gross Investment Income, Operating		3.6	(4.2)
Mortgage Loans
Net Investment Income
Gross Investment Income, Operating		21.6	21.8
Policy Loans
Net Investment Income
Gross Investment Income, Operating		5.3	5.2
Perpetual Preferred Securities
Net Investment Income
Gross Investment Income, Operating		0.0	0.8
Private Equity Partnerships
Net Investment Income
Gross Investment Income, Operating	[1]	23.2	18.3
Other Long-term Investments
Net Investment Income
Gross Investment Income, Operating		4.2	4.3
Short-term Investments
Net Investment Income
Gross Investment Income, Operating		$ 24.8	$ 28.0

[1] 1The net unrealized gain recognized in net investment income for the three months ended March 31, 2026 related to private equity partnerships still held at March 31, 2026 was $29.7 million, reduced by net management fees and partnership expenses of $(6.5) million. The net unrealized gain (loss) recognized in net investment income for the three months ended March 31, 2025 related to private equity partnerships still held at March 31, 2025 was $26.4 million, reduced by net management fees and partnership expense of $(8.1) million. See Note 3 for further discussion of private equity partnerships.