Investments Investment Income (Detail) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Investment Income
Investment Income, Interest and Dividend										$ 2,403.7	$ 2,480.2	$ 2,502.4
Less Investment Expenses										30.6	32.1	35.2
Less Investment Income on PFA Assets										12.4	12.8	13.5
Net Investment Income		$ 593.5	$ 613.2	$ 569.0	$ 585.0	$ 616.3	$ 599.4	$ 624.9	$ 594.7	2,360.7	2,435.3	2,453.7
Fixed Maturity Securities
Net Investment Income
Investment Income, Interest and Dividend										2,164.0	2,213.6	2,239.7
Derivatives
Net Investment Income
Investment Income, Interest and Dividend										78.7	73.4	66.1
Mortgage Loans
Net Investment Income
Investment Income, Interest and Dividend										108.9	103.3	110.1
Policy Loans
Net Investment Income
Investment Income, Interest and Dividend										20.0	19.9	18.6
Perpetual Preferred Equity Securities
Net Investment Income
Investment Income, Interest and Dividend	[1]									(2.1)	5.4	(0.2)
Private Equity Partnerships
Net Investment Income
Investment Income, Interest and Dividend	[2]									19.8	31.7	36.0
Other Long-term Investments
Net Investment Income
Investment Income, Interest and Dividend										3.9	3.9	8.4
Short-term Investments
Net Investment Income
Investment Income, Interest and Dividend										$ 10.5	$ 29.0	$ 23.7

[1] The net unrealized gain (loss) recognized in net investment income for the year ended December 31, 2020 related to perpetual preferred securities still held at December 31, 2020 was $(4.6) million. The net unrealized gain (loss) recognized in net investment income for the year ended December 31, 2019 related to perpetual preferred securities still held at December 31, 2019 was $3.3 million.

[2] The net unrealized gain (loss) recognized in net investment income for the year ended December 31, 2020 related to private equity partnerships still held at December 31, 2020 was $(8.7) million. The net unrealized gain (loss) recognized in net investment income for the year ended December 31, 2019 related to private equity partnerships still held at December 31, 2019 was $6.8 million. See Note 2 for further discussion of private equity partnerships.