Quarterly Financial Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Jan. 31, 2012
Quarterly Financial Data [Abstract]
Total revenues	$ 2,348,219	$ 1,994,907	$ 2,203,977	$ 2,282,435	$ 2,274,129	$ 1,955,539	$ 1,931,877	$ 2,100,185	$ 8,829,538	$ 8,261,730	$ 7,066,822
Total benefits and expenses	2,127,634	1,823,367	2,003,571	2,040,586	1,998,872	1,758,366	1,733,805	1,906,870	7,995,158	7,397,913	6,474,477
Income Loss From Continuing Operations Before Provision For Income Tax	220,585	171,540	200,406	241,849	275,257	197,173	198,072	193,315	834,380	863,817	592,345
Net income	$ 158,531	$ 147,385	$ 132,888	$ 160,816	$ 196,712	$ 128,232	$ 127,019	$ 122,439	$ 599,620	$ 574,402	$ 407,086
Basic earnings per share:
Earnings Per Share, Basic	$ 2.16	$ 2.00	$ 1.80	$ 2.20	$ 2.68	$ 1.75	$ 1.74	$ 1.68	$ 8.15	$ 7.85	$ 5.59
Earnings Per Share, Diluted	$ 2.15	$ 1.98	$ 1.78	$ 2.18	$ 2.62	$ 1.72	$ 1.70	$ 1.64	$ 8.09	$ 7.69	$ 5.55
Dividends declared per share	$ 0.18	$ 0.18	$ 0.12	$ 0.12	$ 0.12	$ 0.12	$ 0.12	$ 0.12	$ 0.60	$ 0.48	$ 0.36
Common Stock Value [Abstract]
Common Stock Price Per Share	$ 52.25	$ 45.95	$ 60.86	$ 62.78	$ 53.71	$ 48.29	$ 45.71	$ 52.52	$ 52.25	$ 53.71
Common Stock Value High	$ 55.51	$ 64.32	$ 63.73	$ 63.14	$ 54.89	$ 51.09	$ 56.49	$ 52.64	$ 55.51	$ 54.89
Common Stock Value Low	$ 44.67	$ 44.51	$ 58.40	$ 53.92	$ 47.30	$ 42.72	$ 44.21	$ 44.89	$ 44.67	$ 47.30
Common stock shares outstanding	73,368	73,267	74,076	73,797	73,363	73,172	73,154	73,103	73,368	73,363
Common Stockholders Of Record												82,599