Quarterly 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
Quarterly Financial Data (Unaudited) [Abstract]
Net earned premium	$ 550,183	$ 544,256	$ 524,251	$ 508,480	$ 509,786	$ 516,166	$ 506,385	$ 509,587	$ 2,127,170	$ 2,041,924	$ 2,037,235
Other revenue	64,738	66,268	60,601	55,228	67,095	59,782	61,738	71,715	246,835	260,330	281,281
Total revenue	614,921	610,524	584,852	563,708	576,881	575,948	568,123	581,302	2,374,005	2,302,254	2,318,516
Loss and loss adjustment expense, net	337,007	380,372	334,282	347,586	290,384	297,138	298,986	326,521	1,399,247	1,213,029	1,215,759
Other expense	165,484	147,360	153,665	153,243	151,460	146,798	149,384	151,756	619,752	599,398	584,206
Total expense	502,491	527,732	487,947	500,829	441,844	443,936	448,370	478,277	2,018,999	1,812,427	1,799,965
Earnings before income taxes	112,430	82,792	96,905	62,879	135,037	132,012	119,753	103,025	355,006	489,827	518,551
Income tax expense	34,092	22,355	27,427	15,889	37,738	38,949	36,373	31,671	99,763	144,731	164,683
Net earnings	$ 78,338	$ 60,437	$ 69,478	$ 46,990	$ 97,299	$ 93,063	$ 83,380	$ 71,354	$ 255,243	$ 345,096	$ 353,868
Earnings per share
Basic	$ 0.74	$ 0.56	$ 0.61	$ 0.41	$ 0.84	$ 0.81	$ 0.72	$ 0.62	$ 2.31	$ 3.00	$ 3.14
Diluted	$ 0.74	$ 0.56	$ 0.61	$ 0.41	$ 0.84	$ 0.81	$ 0.72	$ 0.62	$ 2.30	$ 2.99	$ 3.11
Weighted-average shares outstanding
Basic	104,260	106,919	111,389	113,754	113,834	114,002	113,935	113,668	109,051	113,863	112,200
Diluted	104,356	107,048	111,757	114,106	114,061	114,158	114,188	114,124	109,240	114,077	113,058