Schedule of Unaudited Quarterly Results (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended										12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selected Quarterly Financial Information [Abstract]
Premium Income	$ 2,158.4		$ 2,153.6	$ 2,142.2	$ 2,142.9		$ 2,099.2	$ 2,089.4	$ 2,081.6	$ 2,087.5	$ 8,597.1	$ 8,357.7	$ 8,082.4
Net Investment Income	619.8		609.0	620.5	602.4		617.9	611.4	623.3	606.4	2,451.7	2,459.0	2,481.2
Net Realized Investment Gain (Loss)	11.4		9.8	8.1	11.0		28.4	11.0	5.3	(20.5)	40.3	24.2	(43.8)
Total Revenue	2,839.2		2,819.1	2,822.0	2,806.5		2,796.5	2,763.3	2,761.3	2,725.4	11,286.8	11,046.5	10,731.3
Income Before Income Tax	345.3	[1]	366.8	361.6	330.3	[2]	371.2	332.1	340.2	304.2	1,404.0	1,347.7	1,238.3
Net Income	$ 266.9	[1],[3]	$ 252.3	$ 245.1	$ 229.9	[2]	$ 248.0	$ 236.0	$ 236.8	$ 210.6	$ 994.2	$ 931.4	$ 867.1
Basic Earnings Per Share	$ 1.19	[1],[3]	$ 1.12	$ 1.08	$ 1.00	[2]	$ 1.07	$ 1.01	$ 1.00	$ 0.88	$ 4.39	$ 3.96	$ 3.51
Diluted Earnings Per Share	$ 1.19	[1],[3]	$ 1.12	$ 1.07	$ 1.00	[2]	$ 1.07	$ 1.01	$ 1.00	$ 0.88	$ 4.37	$ 3.95	$ 3.50

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

[2] 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.

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