Quarterly Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Data (Unaudited) [Abstract]
Net earned premium	$ 601,832	$ 586,935	$ 572,248	$ 562,612	$ 560,030	$ 556,668	$ 561,356	$ 561,186	$ 2,323,627	$ 2,239,240	$ 2,242,625
Other revenue	68,175	103,103	71,326	86,318	76,600	79,809	68,075	73,180	328,922	297,664	283,202
Total revenue	670,007	690,038	643,574	648,930	636,630	636,477	629,431	634,366	2,652,549	2,536,904	2,525,827
Loss and loss adjustment expense, net	339,434	320,758	335,897	330,746	305,477	328,530	347,353	341,094	1,326,835	1,322,454	1,338,074
Other expense	166,010	164,623	170,033	163,212	174,770	167,847	157,247	141,876	663,878	641,740	633,326
Total expense	505,444	485,381	505,930	493,958	480,247	496,377	504,600	482,970	1,990,713	1,964,194	1,971,400
Earnings before income taxes	164,563	204,657	137,644	154,972	156,383	140,100	124,831	151,396	661,836	572,710	554,427
Income tax expense	51,534	64,390	40,508	47,061	41,373	41,925	36,669	45,546	203,493	165,513	163,187
Net earnings	$ 113,029	$ 140,267	$ 97,136	$ 107,911	$ 115,010	$ 98,175	$ 88,162	$ 105,850	$ 458,343	$ 407,197	$ 391,240
Earnings per share
Basic	$ 1.17	$ 1.41	$ 0.97	$ 1.08	$ 1.15	$ 0.98	$ 0.88	$ 1.05	$ 4.62	$ 4.05	$ 3.84
Diluted	$ 1.16	$ 1.41	$ 0.97	$ 1.07	$ 1.14	$ 0.98	$ 0.87	$ 1.05	$ 4.61	$ 4.04	$ 3.83
Weighted-average shares outstanding
Basic	95,373	97,909	98,442	98,662	98,770	98,723	98,870	99,056	97,591	98,853	100,176
Diluted	95,636	98,134	98,660	98,913	99,046	98,993	99,121	99,287	97,851	99,113	100,456