Investments Investment Income (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2022	Mar. 31, 2021
Net Investment Income
Investment Income, Interest and Dividend		$ 539.8	$ 559.3
Less Investment Expenses		9.6	7.5
Less Investment Income on PFA Assets		3.0	3.1
Net Investment Income		527.2	548.7
Fixed Maturity Securities
Net Investment Income
Investment Income, Interest and Dividend		455.5	469.8
Derivatives
Net Investment Income
Investment Income, Interest and Dividend		14.4	16.2
Mortgage Loans
Net Investment Income
Investment Income, Interest and Dividend		27.0	26.3
Policy Loans
Net Investment Income
Investment Income, Interest and Dividend		4.7	4.8
Perpetual Preferred Securities
Net Investment Income
Investment Income, Interest and Dividend	[1]	2.9	3.6
Private Equity Partnerships
Net Investment Income
Investment Income, Interest and Dividend	[2]	32.4	35.9
Other Long-term Investments
Net Investment Income
Investment Income, Interest and Dividend		2.1	2.1
Short-term Investments
Net Investment Income
Investment Income, Interest and Dividend		$ 0.8	$ 0.6

[1] 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. The net unrealized gain (loss) recognized in net investment income for the three months ended March 31, 2021 related to perpetual preferred securities still held at March 31, 2021 was $2.7 million.

[2] 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 $16.3 million. The net unrealized gain (loss) recognized in net investment income for the three months ended March 31, 2021 related to private equity partnerships still held at March 31, 2021 was $27.0 million. See Note 3 for further discussion of private equity partnerships.