Investments Investment Income (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2023	Mar. 31, 2022
Net Investment Income
Gross Investment Income, Operating		$ 526.5	$ 539.8
Less Investment Expenses		14.7	9.6
Less Investment Income on PFA Assets		3.0	3.0
Net Investment Income		508.8	527.2
Fixed Maturity Securities
Net Investment Income
Gross Investment Income, Operating		455.1	455.5
Derivatives
Net Investment Income
Gross Investment Income, Operating		11.9	14.4
Mortgage Loans
Net Investment Income
Gross Investment Income, Operating		23.4	27.0
Policy Loans
Net Investment Income
Gross Investment Income, Operating		5.0	4.7
Perpetual Preferred Securities
Net Investment Income
Gross Investment Income, Operating	[1]	0.9	2.9
Private Equity Partnerships
Net Investment Income
Gross Investment Income, Operating	[2]	14.3	32.4
Other Long-term Investments
Net Investment Income
Gross Investment Income, Operating		1.8	2.1
Short-term Investments
Net Investment Income
Gross Investment Income, Operating		$ 14.1	$ 0.8

[1] 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. The net unrealized gain (loss) recognized in net investment income for the three months ended March 31, 2022 related to perpetual preferred securities still held at March 31, 2022 was $2.4 million.

[2] 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 expenses of $(3.5) million. The net unrealized gain (loss) recognized in net investment income for the three months ended March 31, 2022 related to private equity partnerships still held at March 31, 2022 was $36.2 million, reduced by net management fees and partnership expense of $(3.8) million. See Note 3 for further discussion of private equity partnerships.