Quarterly Financial Data (Unaudited) - Unaudited 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
Quarterly Financial Information Disclosure [Abstract]
Premiums earned	$ 86,695	$ 81,596	$ 81,983	$ 79,709	$ 78,741	$ 72,425	$ 69,733	$ 69,790	$ 329,983	$ 290,689	$ 251,015
Net investment income	6,763	6,947	6,649	6,670	6,698	6,801	6,605	6,914	27,029	27,018	26,340
Net realized gains (losses) on investments	710	(654)	(1,291)	24	66	985	137	1,791	(1,211)	2,979	2,228
Total revenues	94,309	88,003	87,511	86,512	85,583	80,374	76,637	78,654	356,335	321,248	280,663
Income before income taxes	25,030	12,447	10,647	11,080	12,674	9,003	4,033	11,433	59,204	37,143	27,357
Net income	17,443	9,699	7,644	8,851	9,226	7,121	3,445	9,561	43,637	29,353	24,181
Net income available to common shareholders	17,364	9,673	7,618	8,836	9,216	7,116	3,443	9,556	43,495	29,331	24,167
Earnings per share:
Basic	$ 0.94	$ 0.53	$ 0.42	$ 0.48	$ 0.51	$ 0.39	$ 0.19	$ 0.53	$ 2.37	$ 1.62	$ 1.32
Diluted	$ 0.92	$ 0.52	$ 0.41	$ 0.47	$ 0.50	$ 0.38	$ 0.19	$ 0.52	$ 2.32	$ 1.58	$ 1.29
Comprehensive income	$ 15,819	$ 8,654	$ 3,584	$ 8,306	$ 8,091	$ 8,570	$ 3,948	$ 9,508	$ 36,363	$ 30,117	$ 26,131
Cash dividends declared per common share	$ 0.08	$ 0.08	$ 0.08	$ 0.08					$ 0.32