Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended											12 Months Ended
	Dec. 31, 2019	[1]	Sep. 30, 2019	[2]	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	[3]	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator
Net Income	$ 296.2		$ 242.0		$ 281.2	$ 280.9	$ 249.1	$ (284.7)		$ 285.5	$ 273.5	$ 1,100.3	$ 523.4	$ 994.2
Denominator
Weighted Average Common Shares - Basic												209,728,900	219,635,600	226,492,400
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards												125,500	423,000.0	842,800
Weighted Average Common Shares - Assuming Dilution												209,854,400	220,058,600	227,335,200
Net Income Per Common Share
Basic	$ 1.44		$ 1.16		$ 1.33	$ 1.31	$ 1.15	$ (1.30)		$ 1.29	$ 1.23	$ 5.25	$ 2.38	$ 4.39
Assuming Dilution	$ 1.44		$ 1.16		$ 1.33	$ 1.31	$ 1.15	$ (1.30)		$ 1.29	$ 1.23	$ 5.24	$ 2.38	$ 4.37

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

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

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