Consolidated Statement of Earnings (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
REVENUE
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
Net investment income									222,634	212,271	203,819
Other operating income									30,448	35,590	44,832
Net realized investment gain									31,148	3,653	12,104
Other-than-temporary impairment credit losses									(1,028)	(4,679)	(425)
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
EXPENSE
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
Policy acquisition costs, net									281,201	266,125	255,136
Other expense	167,070	174,040	161,144	163,635	165,484	147,360	153,665	153,243	665,889	619,752	599,398
Other operating expense									359,060	330,557	322,914
Interest expense									25,628	23,070	21,348
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 common 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