Parent Company Statement of Income (Detail) - USD ($) $ in Millions	3 Months Ended													12 Months Ended
	Dec. 31, 2020	[1]	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
Condensed Income Statements, Captions
Other Income														$ 224.2	$ 221.2	$ 198.2
Total Revenue	$ 4,273.5		$ 2,996.3		$ 3,021.2		$ 2,871.1	$ 3,034.6		$ 2,960.0		$ 3,016.7	$ 2,987.6	13,162.1	11,998.9	11,598.5
Cost Related to Early Retirement of Debt														0.0	27.3	0.0
Income Before Income Tax	124.7	[2],[3],[4]	299.6	[5]	337.6	[6]	202.1	377.4	[7]	299.4	[8]	352.0	353.3	964.0	1,382.1	627.8
Income Tax Benefit														171.0	281.8	104.4
Net Income	$ 135.4	[2],[3],[4],[9]	$ 231.1	[5]	$ 265.5	[6]	$ 161.0	$ 296.2	[7]	$ 242.0	[8]	$ 281.2	$ 280.9	793.0	1,100.3	523.4
Other Comprehensive Income (Loss), Net of Tax														336.9	851.5	(924.2)
Comprehensive Income (Loss)														1,129.9	1,951.8	(400.8)
Parent Company
Condensed Income Statements, Captions
Cash Dividends from Subsidiaries														974.6	1,089.4	1,135.4
Other Income														51.7	63.9	66.6
Total Revenue														1,026.3	1,153.3	1,202.0
Interest and Debt Expense														187.1	173.2	161.4
Cost Related to Early Retirement of Debt														0.0	27.3	0.0
Other Expenses														51.1	53.4	53.4
Operating Expenses, Total														238.2	253.9	214.8
Income Before Income Tax														788.1	899.4	987.2
Income Tax Benefit														(15.3)	(21.5)	(1.7)
Income of Parent Company														803.4	920.9	988.9
Equity in Undistributed Earnings (Loss) of Subsidiaries														(10.4)	179.4	(465.5)
Net Income														793.0	1,100.3	523.4
Other Comprehensive Income (Loss), Net of Tax														336.9	851.5	(924.2)
Comprehensive Income (Loss)														$ 1,129.9	$ 1,951.8	$ (400.8)

[1] Fourth quarter of 2020 net realized investment gain of $1,302.3 million before tax and $1,028.8 million after tax, related to the transfer of investments for the Closed Block individual disability reinsurance transaction.

[2] Fourth quarter of 2020 amortization of the cost of reinsurance related to the Closed Block individual disability reinsurance transaction of $2.6 million before tax and $2.0 million after tax.

[3] Fourth quarter of 2020 impacts from the Closed Block individual disability reinsurance transaction of $1,305.5 million before tax and $1,031.3 million after tax.

[4] Fourth quarter of 2020 reserve increases of $151.5 million and $17.5 million before tax and $119.7 million and $13.8 million after tax related to long-term care and group pension, respectively.

[5] Third quarter of 2020 costs related to organizational design update of $23.3 million before tax and $18.6 million after tax.

[6] Second quarter of 2020 impairment on ROU asset related to an operating lease for office space that we do not plan to continue using to support our general operations of $12.7 million before tax and $10.0 million after tax.

[7] Fourth quarter of 2019 cost related to the early retirement of debt of $2.1 million before tax and $1.7 million after tax.

[8] Third quarter of 2019 cost related to the early retirement of debt of $25.2 million before tax and $19.9 million after tax.

[9] Fourth quarter of 2020 net tax benefit from the Closed Block individual disability reinsurance transaction of $36.5 million.