Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended											12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	[1]	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	[2],[3]	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	[4]	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator
Net Income	$ 249.1	$ (284.7)		$ 285.5	$ 273.5	$ 266.9		$ 252.3	$ 245.1	$ 229.9		$ 523.4	$ 994.2	$ 931.4
Denominator
Weighted Average Common Shares - Basic												219,635,600	226,492,400	235,445,700
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards												423,000	842,800	533,500
Weighted Average Common Shares - Assuming Dilution												220,058,600	227,335,200	235,979,200
Net Income Per Common Share
Basic	$ 1.15	$ (1.30)		$ 1.29	$ 1.23	$ 1.19		$ 1.12	$ 1.08	$ 1.00		$ 2.38	$ 4.39	$ 3.96
Assuming Dilution	$ 1.15	$ (1.30)		$ 1.29	$ 1.23	$ 1.19		$ 1.12	$ 1.07	$ 1.00		$ 2.38	$ 4.37	$ 3.95

[1] Third quarter of 2018 reserve increase of $750.8 million before tax and $593.1 million after tax related to long-term care.

[2] Fourth quarter of 2017 net tax benefit of $31.5 million related to the TCJA.

[3] Fourth quarter of 2017 reserve increase of $39.0 million before tax and $25.4 million after tax related to unclaimed death benefits.

[4] First quarter of 2017 loss from a guaranty fund assessment related to an unaffiliated insurer that was declared insolvent of $20.6 million before tax and $13.4 million after tax.