Summary of Quarterly Financial Data (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Gross written premiums	$ 333,091	$ 312,076	$ 332,128	$ 393,222	$ 321,587	$ 298,742	$ 322,987	$ 343,149	$ 1,370,517	$ 1,286,465	$ 1,108,216
Revenues:
Net written premiums	223,445	196,556	198,469	269,452	212,312	188,046	190,252	243,045	887,922	833,655	753,798
Change in unearned premiums	(3,543)	17,339	7,345	(67,124)	(10,746)	13,216	5,765	(59,926)	(45,983)	(51,691)	(62,153)
Net earned premiums	219,902	213,895	205,814	202,328	201,566	201,262	196,017	183,119	841,939	781,964	691,645
Net investment income	14,254	14,094	14,246	13,657	13,616	13,597	15,777	11,258	56,251	54,248	63,500
Total other-than-temporary impairment losses	(530)	(1,821)		(42)	(208)		(496)	(198)	(2,393)	(902)	(3,617)
Portion of loss recognized in other comprehensive income (before tax)								44		44	1,632
Net other-than-temporary impairment losses recognized in earnings	(530)	(1,821)		(42)	(208)		(496)	(154)	(2,393)	(858)	(1,985)
Net realized gains (losses)	15,768	(988)	3,345	4,814	30,254	4,761	4,217	1,842	22,939	41,074	11,996
Other income (expense)	(665)	(210)	(915)	618	(599)	789	387	911	(1,172)	1,488	1,229
Total revenues	248,729	224,970	222,490	221,375	244,629	220,409	215,902	196,976	917,564	877,916	766,385
Expenses:
Net losses and loss adjustment expenses	131,385	125,086	131,148	131,342	127,191	128,850	123,407	117,985	518,961	497,433	476,997
Commission expenses	30,863	27,685	28,391	26,555	31,259	31,258	29,503	29,450	113,494	121,470	110,437
Other operating expenses	43,826	39,056	40,678	40,874	42,841	40,112	39,819	36,307	164,434	159,079	138,029
Call premium on Senior Notes	17,895								17,895
Interest expense	4,351	2,053	2,052	2,051	2,051	2,049	2,049	2,049	10,507	8,198	8,188
Total expenses	228,320	193,880	202,269	200,822	203,342	202,269	194,778	185,791	825,291	786,180	733,651
Income before income taxes	20,409	31,090	20,221	20,553	41,287	18,140	21,124	11,185	92,273	91,736	32,734
Income tax expense	6,076	9,804	6,284	6,643	13,243	5,225	6,225	3,281	28,807	27,974	7,137
Net income	14,333	21,286	13,937	13,910	28,044	12,915	14,899	7,904	63,466	63,762	25,597
Comprehensive income (loss)	$ 1,200	$ 25,462	$ (24,829)	$ 14,785	$ 2,418	$ 30,760	$ 21,663	$ 13,881	$ 16,618	$ 68,722	$ 55,384
Combined ratio	94.00%	89.80%	97.70%	97.90%	100.20%	99.10%	98.10%	99.80%	94.80%	99.30%	104.70%
Net income (loss) per share:
Basic	$ 1.01	$ 1.50	$ 0.99	$ 0.99	$ 2.00	$ 0.92	$ 1.06	$ 0.57	$ 4.49	$ 4.54	$ 1.71
Diluted	$ 1.00	$ 1.48	$ 0.97	$ 0.97	$ 1.96	$ 0.90	$ 1.05	$ 0.56	$ 4.42	$ 4.45	$ 1.69