Schedule of Unaudited Quarterly Results (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Premium Income	$ 1,937.2	$ 1,929.4	$ 1,927.6	$ 1,921.9	$ 1,888.5	$ 1,881.2	$ 1,875.0	$ 1,869.5	$ 7,716.1	$ 7,514.2	$ 7,431.4
Net Investment Income	643.0	619.2	633.5	619.5	634.6	629.2	637.1	618.7	2,515.2	2,519.6	2,495.5
Net Realized Investment Gain (Loss)	24.6	21.3	(2.1)	12.4	7.4	(23.9)	(3.6)	15.2	56.2	(4.9)	24.7
Total Revenue	2,658.2	2,628.0	2,617.9	2,611.3	2,604.8	2,545.6	2,564.5	2,563.1	10,515.4	10,278.0	10,193.2
Income (Loss) Before Income Tax	322.7	320.4	296.4	310.0	(617.3)	288.7	334.5	327.4	1,249.5	333.3	1,319.9
Net Income	$ 233.9	$ 230.2	$ 216.4	$ 213.9	$ (369.0)	$ 202.0	$ 227.6	$ 223.6	$ 894.4	$ 284.2	$ 878.7
Earnings Per Share, Basic	$ 0.86	$ 0.83	$ 0.76	$ 0.74	$ (1.26)	$ 0.68	$ 0.74	$ 0.72	$ 3.18	$ 0.94	$ 2.70
Earnings Per Share, Diluted	$ 0.85	$ 0.83	$ 0.76	$ 0.73	$ (1.26)	$ 0.68	$ 0.74	$ 0.71	$ 3.17	$ 0.94	$ 2.69