Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended													12 Months Ended
	Dec. 31, 2020	[1],[2],[3],[4],[5]	Sep. 30, 2020	[6]	Jun. 30, 2020	[7]	Mar. 31, 2020	Dec. 31, 2019	[8]	Sep. 30, 2019	[9]	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator
Net Income	$ 135.4		$ 231.1		$ 265.5		$ 161.0	$ 296.2		$ 242.0		$ 281.2	$ 280.9	$ 793.0	$ 1,100.3	$ 523.4
Denominator
Weighted Average Common Shares - Basic														203,642,000.0	209,728,900	219,635,600
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards														113,300	125,500	423,000.0
Weighted Average Common Shares - Assuming Dilution														203,755,300	209,854,400	220,058,600
Net Income Per Common Share
Basic	$ 0.66		$ 1.13		$ 1.30		$ 0.79	$ 1.44		$ 1.16		$ 1.33	$ 1.31	$ 3.89	$ 5.25	$ 2.38
Assuming Dilution	$ 0.66		$ 1.13		$ 1.30		$ 0.79	$ 1.44		$ 1.16		$ 1.33	$ 1.31	$ 3.89	$ 5.24	$ 2.38

[1] 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.

[2] 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.

[3] 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.

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

[5] 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.

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

[7] 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.

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

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