Summary of Quarterly Financial Data (Detail) (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
Gross written premiums	$ 277,901	$ 255,318	$ 278,714	$ 296,283	$ 229,850	$ 233,638	$ 253,568	$ 270,145	$ 1,108,216	$ 987,201	$ 1,044,918
Net written premiums	202,002	175,357	183,363	193,076	141,809	157,807	165,005	189,317	753,798	653,938	701,255
Revenues:
Net earned premiums	191,757	173,633	173,777	152,478	166,158	168,233	161,471	164,069	691,645	659,931	683,363
Net investment income	12,428	16,259	17,429	17,384	17,998	17,839	17,853	17,972	63,500	71,662	75,512
Total other-than-temporary impairment losses	(1,280)	(1,241)	(833)	(263)	(448)	(1,034)	(489)	(251)	(3,617)	(2,222)	(29,265)
Portion of loss recognized in other comprehensive income (before tax)	691	618	301	22	273	365	334	170	1,632	1,142	17,388
Net other-than-temporary impairment losses recognized in earnings	(589)	(623)	(532)	(241)	(175)	(669)	(155)	(81)	(1,985)	(1,080)	(11,877)
Net realized gains (losses)	7,141	3,238	3,006	(1,389)	19,665	4,521	11,020	6,113	11,996	41,319	9,217
Other income (expense)	586	(921)	573	991	2,205	2,767	(899)	1,070	1,229	5,143	6,665
Total revenues	211,323	191,586	194,253	169,223	205,851	192,691	189,290	189,143	766,385	776,975	762,880
Expenses:
Net losses and loss adjustment expenses	136,104	110,242	113,863	116,788	110,022	107,463	99,863	103,807	476,997	421,155	435,998
Commission expenses	30,273	25,934	28,030	26,200	32,935	25,185	25,677	25,316	110,437	109,113	98,908
Other operating expenses	30,688	34,989	35,777	36,575	35,919	34,682	34,513	34,586	138,029	139,700	132,671
Interest expense	2,048	2,047	2,047	2,046	2,045	2,045	2,044	2,044	8,188	8,178	8,455
Total expenses	199,113	173,212	179,717	181,609	180,921	169,375	162,097	165,753	733,651	678,146	676,032
Income (loss) before income taxes	12,210	18,374	14,536	(12,386)	24,930	23,316	27,193	23,390	32,734	98,829	86,848
Income tax expense (benefit)	2,122	4,476	5,032	(4,493)	7,592	7,091	8,223	6,345	7,137	29,251	23,690
Net income (loss)	10,088	13,898	9,504	(7,893)	17,338	16,225	18,970	17,045	25,597	69,578	63,158
Comprehensive income (loss)	$ 23,293	$ 12,686	$ 26,329	$ (6,924)	$ (24,308)	$ 40,023	$ 26,164	$ 24,505	$ 55,384	$ 66,384	$ 113,312
Combined ratio	102.50%	99.10%	101.90%	117.10%	106.30%	97.80%	99.70%	99.10%	104.70%	100.70%	97.20%
Net income (loss) per common share:
Basic	$ 0.71	$ 0.94	$ 0.62	$ (0.50)	$ 1.10	$ 1.03	$ 1.18	$ 1.02	$ 1.71	$ 4.33	$ 3.73
Diluted	$ 0.70	$ 0.92	$ 0.60	$ (0.50)	$ 1.07	$ 1.00	$ 1.16	$ 1.00	$ 1.69	$ 4.24	$ 3.65