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, 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
Quarterly Financial Information Disclosure [Abstract]
Premiums earned	$ 97,070	$ 95,928	$ 93,516	$ 89,233	$ 86,695	$ 81,596	$ 81,983	$ 79,709	$ 375,747	$ 329,983	$ 290,689
Net investment income	7,166	6,495	6,845	6,708	6,763	6,947	6,649	6,670	27,214	27,029	27,018
Net realized gains (losses) on investments	516	(152)	232	101	710	(654)	(1,291)	24	697	(1,211)	2,979
Total revenues	104,886	102,336	100,624	96,173	94,309	88,003	87,511	86,512	404,019	356,335	321,248
Income before income taxes	23,936	19,348	17,061	13,404	25,030	12,447	10,647	11,080	73,749	59,204	37,143
Net income	16,865	13,479	12,773	10,549	17,443	9,699	7,644	8,851	53,666	43,637	29,353
Net income available to common shareholders	16,865	13,479	12,773	10,549	17,364	9,673	7,618	8,836	53,666	43,495	29,331
Earnings per share:
Basic	$ 0.90	$ 0.72	$ 0.69	$ 0.57	$ 0.94	$ 0.53	$ 0.42	$ 0.48	$ 2.88	$ 2.37	$ 1.62
Diluted	$ 0.89	$ 0.71	$ 0.68	$ 0.56	$ 0.92	$ 0.52	$ 0.41	$ 0.47	$ 2.84	$ 2.32	$ 1.58
Comprehensive income	$ 17,597	$ 13,596	$ 15,789	$ 13,789	$ 15,819	$ 8,654	$ 3,584	$ 8,306	$ 60,771	$ 36,363	$ 30,117
Extraordinary cash dividends declared per common share	$ 1.000			$ 0.500					$ 1.50	$ 0.00
Cash dividends declared per common share	$ 0.12	$ 0.12	$ 0.12	$ 0.12	$ 0.08	$ 0.08	$ 0.08	$ 0.08	$ 0.48	$ 0.32