Consolidated Statements of Income (USD $) In Thousands, except 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
Gross written premiums	$ 333,299	$ 327,469	$ 348,795	$ 422,790	$ 333,091	$ 312,076	$ 332,128	$ 393,222	$ 1,432,353	$ 1,370,517	$ 1,286,465
Revenues:
Net written premiums	228,007	228,417	231,864	311,850	223,445	196,556	198,469	269,452	1,000,138	887,922	833,655
Change in unearned premiums	6,165	16,950	(780)	(86,578)	(3,543)	17,339	7,345	(67,124)	(64,243)	(45,983)	(51,691)
Net earned premiums	234,172	245,367	231,084	225,272	219,902	213,895	205,814	202,328	935,895	841,939	781,964
Net investment income	16,071	15,839	15,648	16,610	14,254	14,094	14,246	13,657	64,168	56,251	54,248
Total other-than-temporary impairment losses	(21)	158			(530)	(1,821)		(42)	137	(2,393)	(902)
Portion of loss recognized in other comprehensive income (before tax)	21	(158)							(137)		44
Net other-than-temporary impairment losses recognized in earnings					(530)	(1,821)		(42)		(2,393)	(858)
Net realized gains (losses)	788	6,718	4,473	833	15,768	(988)	3,345	4,814	12,812	22,939	41,074
Other income (expense)	586	1,336	(1,665)	10,399	(665)	(210)	(915)	618	10,656	(1,172)	1,488
Total revenues	251,617	269,260	249,540	253,114	248,729	224,970	222,490	221,375	1,023,531	917,564	877,916
Expenses:
Net losses and loss adjustment expenses	134,658	135,284	140,220	135,067	131,385	125,086	131,148	131,342	545,229	518,961	497,433
Commission expenses	33,708	33,943	32,150	25,727	30,863	27,685	28,391	26,555	125,528	113,494	121,470
Other operating expenses	51,299	50,388	47,992	47,146	43,826	39,056	40,678	40,874	196,825	164,434	159,079
Call premium on Senior Notes					17,895					17,895
Interest expense	3,854	3,388	4,319	3,852	4,351	2,053	2,052	2,051	15,413	10,507	8,198
Total expenses	223,519	223,003	224,681	211,792	228,320	193,880	202,269	200,822	882,995	825,291	786,180
Income (loss) before income taxes	28,098	46,257	24,859	41,322	20,409	31,090	20,221	20,553	140,536	92,273	91,736
Income tax expense (benefit)	8,823	15,032	7,998	13,354	6,076	9,804	6,284	6,643	45,207	28,807	27,974
Net income	$ 19,275	$ 31,225	$ 16,861	$ 27,968	$ 14,333	$ 21,286	$ 13,937	$ 13,910	$ 95,329	$ 63,466	$ 63,762
Net income per common share:
Basic	$ 1.35	$ 2.19	$ 1.18	$ 1.96	$ 1.01	$ 1.50	$ 0.99	$ 0.99	$ 6.69	$ 4.49	$ 4.54
Diluted	$ 1.31	$ 2.14	$ 1.17	$ 1.94	$ 1.00	$ 1.48	$ 0.97	$ 0.97	$ 6.51	$ 4.42	$ 4.45
Average common shares outstanding:
Basic									14,259,768	14,133,925	14,052,311
Diluted									14,646,369	14,345,553	14,327,820