Investments Investment Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022		Dec. 31, 2021		Dec. 31, 2020
Net Investment Income
Investment Income, Interest and Dividend	$ 2,173.6		$ 2,260.6		$ 2,403.7
Less Investment Expenses	39.4		35.1		30.6
Less Investment Income on PFA Assets	12.0		12.3		12.4
Net Investment Income	2,122.2		2,213.2		2,360.7
Fixed Maturity Securities
Net Investment Income
Investment Income, Interest and Dividend	1,849.8		1,888.2		2,164.0
Derivatives
Net Investment Income
Investment Income, Interest and Dividend	57.8		68.6		78.7
Mortgage Loans
Net Investment Income
Investment Income, Interest and Dividend	101.5		105.0		108.9
Policy Loans
Net Investment Income
Investment Income, Interest and Dividend	20.0		19.7		20.0
Perpetual Preferred Securities
Net Investment Income
Investment Income, Interest and Dividend	5.0	[1]	6.9	[1]	(2.1)
Private Equity Partnerships
Net Investment Income
Investment Income, Interest and Dividend	110.1		165.4		19.8
Other Long-term Investments
Net Investment Income
Investment Income, Interest and Dividend	9.4		5.5		3.9
Short-term Investments
Net Investment Income
Investment Income, Interest and Dividend	$ 20.0		$ 1.3		$ 10.5

[1] The net unrealized gain (loss) recognized in net investment income for the year ended December 31, 2022 related to perpetual preferred securities still held at December 31, 2022 was $2.8 million. The net unrealized gain (loss) recognized in net investment income for the years ended December 31, 2021 and 2020 related to perpetual preferred securities still held at year-end was $4.4 million and $(4.6) million, respectively