Schedule of Unaudited Quarterly Results (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Premium Income	$ 1,890.7	$ 1,897.3	$ 1,905.8	$ 1,930.9	$ 1,937.2	$ 1,929.4	$ 1,927.6	$ 1,921.9	$ 7,624.7	$ 7,716.1	$ 7,514.2
Net Investment Income	629.4	615.5	626.1	621.1	643.0	619.2	633.5	619.5	2,492.1	2,515.2	2,519.6
Net Realized Investment Gain (Loss)	9.3	(26.1)	13.3	10.3	24.6	21.3	(2.1)	12.4	6.8	56.2	(4.9)
Total Revenue	2,586.2	2,540.9	2,601.9	2,624.8	2,658.2	2,628.0	2,617.9	2,611.3	10,353.8	10,515.4	10,278.0
Income Before Income Tax	305.8	284.1	311.5	303.8	322.7	320.4	296.4	310.0	1,205.2	1,249.5	333.3
Net Income	$ 221.2	$ 205.7	$ 218.6	$ 212.6	$ 233.9	$ 230.2	$ 216.4	$ 213.9	$ 858.1	$ 894.4	$ 284.2
Earnings Per Share, Basic	$ 0.85	$ 0.78	$ 0.82	$ 0.79	$ 0.86	$ 0.83	$ 0.76	$ 0.74	$ 3.24	$ 3.18	$ 0.94
Earnings Per Share, Diluted	$ 0.84	$ 0.78	$ 0.82	$ 0.79	$ 0.85	$ 0.83	$ 0.76	$ 0.73	$ 3.23	$ 3.17	$ 0.94