Investments Investment Income (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Net Investment Income
Investment Income, Interest and Dividend		$ 596.7	$ 607.0
Less Investment Expenses		8.5	9.0
Less Investment Income on PFA Assets		3.2	3.3
Net Investment Income		585.0	594.7
Fixed Maturity Securities
Net Investment Income
Investment Income, Interest and Dividend		539.8	544.9
Derivatives
Net Investment Income
Investment Income, Interest and Dividend		20.2	17.9
Mortgage Loans
Net Investment Income
Investment Income, Interest and Dividend		29.0	26.7
Policy Loans
Net Investment Income
Investment Income, Interest and Dividend		4.8	4.7
Perpetual Preferred Securities
Net Investment Income
Investment Income, Interest and Dividend	[1]	(17.1)	3.7
Private Equity Partnerships
Net Investment Income
Investment Income, Interest and Dividend	[2]	10.4	2.2
Other Long-term Investments
Net Investment Income
Investment Income, Interest and Dividend		3.2	(0.1)
Short-term Investments
Net Investment Income
Investment Income, Interest and Dividend		$ 6.4	$ 7.0

[1] 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. The net unrealized gain recognized in net investment income for the three months ended March 31, 2019 related to perpetual preferred securities still held at March 31, 2019 was $3.1 million.

[2] 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. The net unrealized loss recognized in net investment income for the three months ended March 31, 2019 related to private equity partnerships still held at March 31, 2019 was $2.7 million.