Schedule of Unaudited Quarterly Results (Detail) - USD ($) $ / shares in Units, $ 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
Selected Quarterly Financial Information [Abstract]
Premium Income	$ 2,319.9		$ 2,318.1		$ 2,368.7		$ 2,371.4	$ 2,352.6		$ 2,331.2		$ 2,343.1	$ 2,338.7	$ 9,378.1	$ 9,365.6	$ 8,986.1
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
Net Realized Investment Gain (Loss)	1,304.9	[1]	4.4		33.8		(144.0)	9.2		(26.2)		(7.3)	1.1	1,199.1	(23.2)	(39.5)
Total Revenue	4,273.5	[1]	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
Income Before Income Tax	124.7	[1],[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
Net Income	$ 135.4	[1],[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
Basic Earnings Per Share	$ 0.66	[1],[2],[3],[4],[9]	$ 1.13	[5]	$ 1.30	[6]	$ 0.79	$ 1.44	[7]	$ 1.16	[8]	$ 1.33	$ 1.31	$ 3.89	$ 5.25	$ 2.38
Diluted Earnings Per Share	$ 0.66	[1],[2],[3],[4],[9]	$ 1.13	[5]	$ 1.30	[6]	$ 0.79	$ 1.44	[7]	$ 1.16	[8]	$ 1.33	$ 1.31	$ 3.89	$ 5.24	$ 2.38

[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.