Investments Investment Income (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Net Investment Income
Gross Investment Income, Operating		$ 562.2	$ 550.0	$ 1,095.3	$ 1,076.5
Less Investment Expenses		14.2	15.8	30.8	30.5
Less Investment Income on PFA Assets		2.9	3.1	5.9	6.1
Net Investment Income		545.1	531.1	1,058.6	1,039.9
Fixed Maturity Securities
Net Investment Income
Gross Investment Income, Operating		469.2	470.7	923.1	925.8
Derivatives
Net Investment Income
Gross Investment Income, Operating		8.7	11.0	16.9	22.9
Mortgage Loans
Net Investment Income
Gross Investment Income, Operating		22.1	23.5	44.6	46.9
Policy Loans
Net Investment Income
Gross Investment Income, Operating		5.2	5.2	10.6	10.2
Perpetual Preferred Securities
Net Investment Income
Gross Investment Income, Operating	[1]	0.1	0.9	0.3	1.8
Unrealized Gain (Loss) on Investments		(0.1)	(0.2)	(0.2)	0.5
Private Equity Partnerships
Net Investment Income
Gross Investment Income, Operating	[2]	32.7	19.8	53.0	34.1
Unrealized Gain (Loss) on Investments		37.3	25.1	62.3	42.9
Management Fee Expense		(4.6)	(5.3)	(9.3)	(8.8)
Other Long-term Investments
Net Investment Income
Gross Investment Income, Operating		2.4	2.7	5.1	4.5
Short-term Investments
Net Investment Income
Gross Investment Income, Operating		$ 21.8	$ 16.2	$ 41.7	$ 30.3

[1] The net unrealized gain (loss) recognized in net investment income for the three and six months ended June 30, 2024 related to perpetual preferred securities still held at June 30, 2024 was $(0.1) million and $(0.2) million, respectively. The net unrealized gain (loss) recognized in net investment income for the three and six months ended June 30, 2023 related to perpetual preferred securities still held at June 30, 2023 was $(0.2) million and $0.5 million, respectively.

[2] 2The net unrealized gain recognized in net investment income for the three and six months ended June 30, 2024 related to private equity partnerships still held at June 30, 2024 was $37.3 million and $62.3 million, respectively, reduced by net management fees and partnership expenses of $(4.6) million and $(9.3) million, respectively. The net unrealized gain recognized in net investment income for the three and six months ended June 30, 2023 related to private equity partnerships still held at June 30, 2023 was $25.1 million and $42.9 million, respectively, reduced by net management fees and partnership expenses of $(5.3) million and $(8.8) million, respectively. See Note 3 for further discussion of private equity partnerships.