Schedule of Unaudited Quarterly Results (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Selected Quarterly Financial Information [Abstract]
Premium Income	$ 1,967.9	$ 1,947.2	$ 1,943.6	$ 1,938.5	$ 1,890.7	$ 1,897.3	$ 1,905.8	$ 1,930.9	$ 7,797.2	$ 7,624.7	$ 7,716.1
Net Investment Income	629.4	606.4	629.1	612.5	629.4	615.5	626.1	621.1	2,477.4	2,492.1	2,515.2
Net Realized Investment Gain	(17.3)	1.2	25.9	6.3	9.3	(26.1)	13.3	10.3	16.1	6.8	56.2
Total Revenue	2,635.7	2,609.4	2,653.5	2,611.1	2,586.2	2,540.9	2,601.9	2,624.8	10,509.7	10,353.8	10,515.4
Income Before Income Tax	(457.9)	312.1	346.5	326.5	305.8	284.1	311.5	303.8	527.2	1,205.2	1,249.5
Net Income	$ (279.1)	$ 221.1	$ 242.5	$ 228.9	$ 221.2	$ 205.7	$ 218.6	$ 212.6	$ 413.4	$ 858.1	$ 894.4
Earnings Per Share, Basic	$ (1.11)	$ 0.87	$ 0.94	$ 0.88	$ 0.85	$ 0.78	$ 0.82	$ 0.79	$ 1.62	$ 3.24	$ 3.18
Earnings Per Share, Diluted	$ (1.11)	$ 0.87	$ 0.94	$ 0.88	$ 0.84	$ 0.78	$ 0.82	$ 0.79	$ 1.61	$ 3.23	$ 3.17