Schedule of Unaudited Quarterly Results (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Selected Quarterly Financial Information [Abstract]
Premium Income	$ 2,037.8	$ 2,020.8	$ 2,017.5	$ 2,006.3	$ 1,967.9	$ 1,947.2	$ 1,943.6	$ 1,938.5	$ 8,082.4	$ 7,797.2	$ 7,624.7
Net Investment Income	636.4	612.1	630.7	602.0	632.7	611.3	632.4	615.8	2,481.2	2,492.2	2,506.9
Net Realized Investment Gain (Loss)	(2.7)	(26.6)	0.8	(15.3)	(17.3)	1.2	25.9	6.3	(43.8)	16.1	6.8
Total Revenue	2,722.4	2,657.8	2,703.7	2,647.4	2,639.0	2,614.3	2,656.8	2,614.4	10,731.3	10,524.5	10,368.6
Income (Loss) Before Income Tax	316.6	298.9	319.8	303.0	(454.6)	317.0	349.8	329.8	1,238.3	542.0	1,220.0
Net Income (Loss)	$ 226.1	$ 203.8	$ 224.3	$ 212.9	$ (282.2)	$ 219.1	$ 239.4	$ 225.8	$ 867.1	$ 402.1	$ 847.0
Basic Earnings Per Share	$ 0.93	$ 0.83	$ 0.90	$ 0.85	$ (1.12)	$ 0.86	$ 0.93	$ 0.87	$ 3.51	$ 1.57	$ 3.20
Diluted Earnings Per Share	$ 0.93	$ 0.83	$ 0.90	$ 0.84	$ (1.12)	$ 0.86	$ 0.93	$ 0.86	$ 3.50	$ 1.57	$ 3.19