Investments Investment Income (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net Investment Income
Investment Income, Interest and Dividend		$ 623.5	$ 609.9	$ 1,799.8	$ 1,852.3
Less Investment Expenses		7.2	7.3	23.3	23.6
Less Investment Income on PFA Assets		3.1	3.2	9.3	9.7
Net Investment Income		613.2	599.4	1,767.2	1,819.0
Fixed Maturity Securities
Net Investment Income
Investment Income, Interest and Dividend		560.8	546.7	1,643.1	1,656.3
Derivatives
Net Investment Income
Investment Income, Interest and Dividend		18.7	19.1	59.6	55.0
Mortgage Loans
Net Investment Income
Investment Income, Interest and Dividend		26.1	25.4	83.1	77.0
Policy Loans
Net Investment Income
Investment Income, Interest and Dividend		5.1	5.0	14.9	14.5
Perpetual Preferred Securities
Net Investment Income
Investment Income, Interest and Dividend	[1]	(0.2)	0.1	(6.7)	4.8
Private Equity Partnerships
Net Investment Income
Investment Income, Interest and Dividend	[2]	11.3	4.7	(9.6)	19.5
Other Long-term Investments
Net Investment Income
Investment Income, Interest and Dividend		0.4	0.2	5.3	2.8
Short-term Investments
Net Investment Income
Investment Income, Interest and Dividend		$ 1.3	$ 8.7	$ 10.1	$ 22.4

[1] The net unrealized gain (loss) recognized in net investment income for the three and nine months ended September 30, 2020 related to perpetual preferred securities still held at September 30, 2020 was $(0.8) million and $(8.7) million, respectively. The net unrealized gain (loss) recognized in net investment income for the three and nine months ended September 30, 2019 related to perpetual preferred securities still held at September 30, 2019 was $0.3 million and $3.3 million, respectively.

[2] The net unrealized gain (loss) recognized in net investment income for the three and nine months ended September 30, 2020 related to private equity partnerships still held at September 30, 2020 was $8.5 million and $(27.1) million, respectively. The net unrealized gain (loss) recognized in net investment income for the three and nine months ended September 30, 2019 related to private equity partnerships still held at September 30, 2019 was $(1.1) million and $2.6 million, respectively. See Note 3 for further discussion of private equity partnerships.