Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended										12 Months Ended
	Dec. 31, 2017	[1],[2]	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	[3]	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator
Net Income	$ 266.9		$ 252.3	$ 245.1	$ 229.9		$ 248.0	$ 236.0	$ 236.8	$ 210.6	$ 994.2	$ 931.4	$ 867.1
Denominator
Weighted Average Common Shares - Basic											226,492,400	235,445,700	246,986,700
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards											842,800	533,500	868,000
Weighted Average Common Shares - Assuming Dilution											227,335,200	235,979,200	247,854,700
Net Income Per Common Share
Basic	$ 1.19		$ 1.12	$ 1.08	$ 1.00		$ 1.07	$ 1.01	$ 1.00	$ 0.88	$ 4.39	$ 3.96	$ 3.51
Assuming Dilution	$ 1.19		$ 1.12	$ 1.07	$ 1.00		$ 1.07	$ 1.01	$ 1.00	$ 0.88	$ 4.37	$ 3.95	$ 3.50

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

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

[3] 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.