Investments Investment Income (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Net Investment Income
Investment Income, Interest and Dividend		$ 524.6	$ 564.3	$ 1,635.8	$ 1,697.9
Less Investment Expenses		10.0	10.8	29.1	26.1
Less Investment Income on PFA Assets		3.0	3.3	8.9	9.4
Net Investment Income		511.6	550.2	1,597.8	1,662.4
Fixed Maturity Securities
Net Investment Income
Investment Income, Interest and Dividend		456.7	478.5	1,379.9	1,419.7
Derivatives
Net Investment Income
Investment Income, Interest and Dividend		14.9	15.2	45.1	49.1
Mortgage Loans
Net Investment Income
Investment Income, Interest and Dividend		24.7	26.5	77.6	77.5
Policy Loans
Net Investment Income
Investment Income, Interest and Dividend		5.1	4.9	14.8	14.7
Perpetual Preferred Securities
Net Investment Income
Investment Income, Interest and Dividend	[1]	1.1	0.4	3.2	6.0
Unrealized Gain (Loss) on Investments		0.5	(0.2)	1.5	4.0
Private Equity Partnerships
Net Investment Income
Investment Income, Interest and Dividend		12.6	38.3	98.6	126.1
Other Long-term Investments
Net Investment Income
Investment Income, Interest and Dividend		3.6	0.3	7.4	3.7
Short-term Investments
Net Investment Income
Investment Income, Interest and Dividend		$ 5.9	$ 0.2	$ 9.2	$ 1.1

[1] The net unrealized gain 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. The net unrealized gain (loss) recognized in net investment income for the three and nine months ended September 30, 2021 related to perpetual preferred securities still held at September 30, 2021 was $(0.2) million and $4.0 million, respectively.