Investments Investment Income (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Net Investment Income
Investment Income, Interest and Dividend		$ 559.3	$ 596.7
Less Investment Expenses		7.5	8.5
Less Investment Income on PFA Assets		3.1	3.2
Net Investment Income		548.7	585.0
Fixed Maturity Securities
Net Investment Income
Investment Income, Interest and Dividend		469.8	539.8
Derivatives
Net Investment Income
Investment Income, Interest and Dividend		16.2	20.2
Mortgage Loans
Net Investment Income
Investment Income, Interest and Dividend		26.3	29.0
Policy Loans
Net Investment Income
Investment Income, Interest and Dividend		4.8	4.8
Perpetual Preferred Securities
Net Investment Income
Investment Income, Interest and Dividend	[1]	3.6	(17.1)
Private Equity Partnerships
Net Investment Income
Investment Income, Interest and Dividend	[2]	35.9	10.4
Other Long-term Investments
Net Investment Income
Investment Income, Interest and Dividend		2.1	3.2
Short-term Investments
Net Investment Income
Investment Income, Interest and Dividend		$ 0.6	$ 6.4

[1] 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. The net unrealized loss recognized in net investment income for the three months ended March 31, 2020 related to perpetual preferred securities still held at March 31, 2020 was $17.9 million.

[2] The net unrealized gain 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. The net unrealized gain recognized in net investment income for the three months ended March 31, 2020 related to private equity partnerships still held at March 31, 2020 was $5.4 million. See Note 3 for further discussion of private equity partnerships.