Quarterly Information (Details) (USD $) In Thousands, 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
Quarterly Financial Data (Unaudited) [Abstract]
Net earned premium	$ 566,503	$ 563,650	$ 565,331	$ 547,141	$ 550,183	$ 544,256	$ 524,251	$ 508,480	$ 2,242,625	$ 2,127,170	$ 2,041,924
Other revenue	85,840	68,023	66,957	62,382	64,738	66,268	60,601	55,228	283,202	246,835	260,330
Total revenue	652,343	631,673	632,288	609,523	614,921	610,524	584,852	563,708	2,525,827	2,374,005	2,302,254
Loss and loss adjustment expense, net	335,744	304,014	336,825	328,928	337,007	380,372	334,282	347,586	1,305,511	1,399,247	1,213,029
Other expense	167,070	174,040	161,144	163,635	165,484	147,360	153,665	153,243	665,889	619,752	599,398
Total expense	502,814	478,054	497,969	492,563	502,491	527,732	487,947	500,829	1,971,400	2,018,999	1,812,427
Earnings before income taxes	149,529	153,619	134,319	116,960	112,430	82,792	96,905	62,879	554,427	355,006	489,827
Income tax expense	41,428	46,557	40,826	34,376	34,092	22,355	27,427	15,889	163,187	99,763	144,731
Net earnings	$ 108,101	$ 107,062	$ 93,493	$ 82,584	$ 78,338	$ 60,437	$ 69,478	$ 46,990	$ 391,240	$ 255,243	$ 345,096
Earnings per share
Basic	$ 1.07	$ 1.06	$ 0.92	$ 0.80	$ 0.74	$ 0.56	$ 0.61	$ 0.41	$ 3.84	$ 2.31	$ 3.00
Diluted	$ 1.06	$ 1.05	$ 0.92	$ 0.79	$ 0.74	$ 0.56	$ 0.61	$ 0.41	$ 3.83	$ 2.30	$ 2.99
Weighted-average shares outstanding
Basic	99,686	99,424	99,563	102,034	104,260	106,919	111,389	113,754	100,176	109,051	113,863
Diluted	99,926	99,700	99,851	102,193	104,356	107,048	111,757	114,106	100,456	109,240	114,077