Investments Investment Income (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Net Investment Income
Gross Investment Income, Operating		$ 542.5	$ 524.6	$ 1,619.0	$ 1,635.8
Less Investment Expenses		13.5	10.0	44.0	29.1
Less Investment Income on PFA Assets		3.0	3.0	9.1	8.9
Net Investment Income		526.0	511.6	1,565.9	1,597.8
Fixed Maturity Securities
Net Investment Income
Gross Investment Income, Operating		457.9	456.7	1,383.7	1,379.9
Derivatives
Net Investment Income
Gross Investment Income, Operating		11.6	14.9	34.5	45.1
Mortgage Loans
Net Investment Income
Gross Investment Income, Operating		22.9	24.7	69.8	77.6
Policy Loans
Net Investment Income
Gross Investment Income, Operating		5.4	5.1	15.6	14.8
Perpetual Preferred Securities
Net Investment Income
Gross Investment Income, Operating	[1]	0.4	1.1	2.2	3.2
Unrealized Gain (Loss) on Investments		(0.4)	0.5	0.1	1.5
Private Equity Partnerships
Net Investment Income
Gross Investment Income, Operating	[2]	22.6	12.6	56.7	98.6
Unrealized Gain (Loss) on Investments		29.7	14.3	72.6	106.8
Management Fee Expense		(7.1)	(1.8)	(15.9)	(8.3)
Other Long-term Investments
Net Investment Income
Gross Investment Income, Operating		2.6	3.6	7.1	7.4
Short-term Investments
Net Investment Income
Gross Investment Income, Operating		$ 19.1	$ 5.9	$ 49.4	$ 9.2

[1] The net unrealized gain (loss) recognized in net investment income for the three and nine months ended September 30, 2023 related to perpetual preferred securities still held at September 30, 2023 was $(0.4) million and $0.1 million, respectively. The net unrealized gain (loss) recognized in net investment income for the three and nine months ended September 30, 2022 related to perpetual preferred securities still held at September 30, 2022 was $0.5 million and $1.5 million, respectively.

[2] 2The net unrealized gain recognized in net investment income for the three and nine months ended September 30, 2023 related to private equity partnerships still held at September 30, 2023 was $29.7 million and $72.6 million, respectively, reduced by net management fees and partnership expenses of $(7.1) million and $(15.9) million, respectively. The net unrealized gain recognized in net investment income for the three and nine months ended September 30, 2022 related to private equity partnerships still held at September 30, 2022 was $14.3 million and $106.8 million, respectively, reduced by net management fees and partnership expenses of $(1.8) million and $(8.3) million, respectively. See Note 3 for further discussion of private equity partnerships.