Summary of Quarterly Financial Data (Detail) (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
Gross written premiums	$ 321,587	$ 298,742	$ 322,987	$ 343,149	$ 277,901	$ 255,318	$ 278,714	$ 296,283	$ 1,286,465	$ 1,108,216	$ 987,201
Revenues:
Net written premiums	212,312	188,046	190,252	243,045	202,002	175,357	183,363	193,076	833,655	753,798	653,938
Change in unearned premiums	(10,746)	13,216	5,765	(59,926)	(10,245)	(1,724)	(9,586)	(40,598)	(51,691)	(62,153)	5,993
Net earned premiums	201,566	201,262	196,017	183,119	191,757	173,633	173,777	152,478	781,964	691,645	659,931
Net investment income	13,616	13,597	15,777	11,258	12,428	16,259	17,429	17,384	54,248	63,500	71,662
Total other-than-temporary impairment losses	(208)		(496)	(198)	(1,280)	(1,241)	(833)	(263)	(902)	(3,617)	(2,222)
Portion of loss recognized in other comprehensive income (before tax)				44	691	618	301	22	44	1,632	1,142
Net other-than-temporary impairment losses recognized in earnings	(208)		(496)	(154)	(589)	(623)	(532)	(241)	(858)	(1,985)	(1,080)
Net realized gains (losses)	30,254	4,761	4,217	1,842	7,141	3,238	3,006	(1,389)	41,074	11,996	41,319
Other income (expense)	(599)	789	387	911	586	(921)	573	991	1,488	1,229	5,143
Total revenues	244,629	220,409	215,902	196,976	211,323	191,586	194,253	169,223	877,916	766,385	776,975
Expenses:
Net losses and loss adjustment expenses	127,191	128,850	123,407	117,985	136,104	110,242	113,863	116,788	497,433	476,997	421,155
Commission expenses	31,259	31,258	29,503	29,450	30,273	25,934	28,030	26,200	121,470	110,437	109,113
Other operating expenses	42,841	40,112	39,819	36,307	30,688	34,989	35,777	36,575	159,079	138,029	139,700
Interest expense	2,051	2,049	2,049	2,049	2,048	2,047	2,047	2,046	8,198	8,188	8,178
Total expenses	203,342	202,269	194,778	185,791	199,113	173,212	179,717	181,609	786,180	733,651	678,146
Income (loss) before income taxes	41,287	18,140	21,124	11,185	12,210	18,374	14,536	(12,386)	91,736	32,734	98,829
Income tax expense (benefit)	13,243	5,225	6,225	3,281	2,122	4,476	5,032	(4,493)	27,974	7,137	29,251
Net income (loss)	28,044	12,915	14,899	7,904	10,088	13,898	9,504	(7,893)	63,762	25,597	69,578
Comprehensive income (loss)	$ 2,418	$ 30,760	$ 21,663	$ 13,881	$ 23,293	$ 12,686	$ 26,329	$ (6,924)	$ 68,722	$ 55,384	$ 66,384
Combined ratio	100.20%	99.10%	98.10%	99.80%	102.50%	99.10%	101.90%	117.10%	99.30%	104.70%	100.70%
Net income (loss) per common share:
Basic	$ 2.00	$ 0.92	$ 1.06	$ 0.57	$ 0.71	$ 0.94	$ 0.62	$ (0.50)	$ 4.54	$ 1.71	$ 4.33
Diluted	$ 1.96	$ 0.90	$ 1.05	$ 0.56	$ 0.70	$ 0.92	$ 0.60	$ (0.50)	$ 4.45	$ 1.69	$ 4.24