Schedule of Unaudited Quarterly Results (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Selected Quarterly Financial Information [Abstract]
Premium Income	$ 2,099.2	$ 2,089.4	$ 2,081.6	$ 2,087.5	$ 2,037.8	$ 2,020.8	$ 2,017.5	$ 2,006.3	$ 8,357.7	$ 8,082.4	$ 7,797.2
Net Investment Income	617.9	611.4	623.3	606.4	636.4	612.1	630.7	602.0	2,459.0	2,481.2	2,492.2
Net Realized Investment Gain (Loss)	28.4	11.0	5.3	(20.5)	(2.7)	(26.6)	0.8	(15.3)	24.2	(43.8)	16.1
Total Revenue	2,796.5	2,763.3	2,761.3	2,725.4	2,722.4	2,657.8	2,703.7	2,647.4	11,046.5	10,731.3	10,524.5
Income Before Income Tax	371.2	332.1	340.2	304.2	316.6	298.9	319.8	303.0	1,347.7	1,238.3	542.0
Net Income	$ 248.0	$ 236.0	$ 236.8	$ 210.6	$ 226.1	$ 203.8	$ 224.3	$ 212.9	$ 931.4	$ 867.1	$ 402.1
Basic Earnings Per Share	$ 1.07	$ 1.01	$ 1.00	$ 0.88	$ 0.93	$ 0.83	$ 0.90	$ 0.85	$ 3.96	$ 3.51	$ 1.57
Diluted Earnings Per Share	$ 1.07	$ 1.01	$ 1.00	$ 0.88	$ 0.93	$ 0.83	$ 0.90	$ 0.84	$ 3.95	$ 3.50	$ 1.57