Investments Investment Income (Detail) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net Investment Income
Investment Income, Interest and Dividend										$ 2,480.2	$ 2,502.4	$ 2,498.6
Less Investment Expenses										32.1	35.2	32.8
Less Investment Income on PFA Assets										12.8	13.5	14.1
Net Investment Income		$ 616.3	$ 599.4	$ 624.9	$ 594.7	$ 608.6	$ 619.2	$ 623.6	$ 602.3	2,435.3	2,453.7	2,451.7
Fixed Maturity Securities
Net Investment Income
Investment Income, Interest and Dividend										2,213.6	2,239.7	2,271.8
Derivatives
Net Investment Income
Investment Income, Interest and Dividend										73.4	66.1	56.4
Mortgage Loans
Net Investment Income
Investment Income, Interest and Dividend										103.3	110.1	104.2
Policy Loans
Net Investment Income
Investment Income, Interest and Dividend										19.9	18.6	17.9
Equity Securities
Net Investment Income
Investment Income, Interest and Dividend	[1]									5.6	(0.2)	1.5
Partnership Interest
Net Investment Income
Investment Income, Interest and Dividend	[2]									31.7	36.0	25.5
Miscellaneous Long-term Investments
Net Investment Income
Investment Income, Interest and Dividend										3.7	8.4	10.0
Short-term Investments
Net Investment Income
Investment Income, Interest and Dividend										$ 29.0	$ 23.7	$ 11.3

[1] The net unrealized gain recognized in net investment income for the year ended December 31, 2019 related to equity securities still held at December 31, 2019 was $3.3 million. The net unrealized loss recognized in net investment income for the year ended December 31, 2018 related to equity securities still held at December 31, 2018 was $3.9 million.

[2] The net unrealized gain 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. The net unrealized gain recognized in net investment income for the year ended December 31, 2018 related to private equity partnerships still held at December 31, 2018 was $7.1 million.