Quarterly Financial Data (Unaudited) - Unaudited Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Net premiums earned	$ 74,702	$ 74,771	$ 75,964	$ 78,990	$ 82,277	$ 82,712	$ 82,951	$ 84,948	$ 304,427	$ 332,888	$ 350,326
Net investment income	7,228	7,063	7,324	7,749	8,035	8,264	8,169	8,015	29,364	32,483	30,452
Net realized gains (losses) on investments	(332)	309	163	992	(53)	(4)	(82)	59	1,132	(80)	(1,536)
Net unrealized gains (losses) on equity securities	6,553	844	5,570	(8,763)	1,593	365	642	2,158	4,204	4,758	(2,088)
Total revenues	88,221	83,005	89,082	79,169	91,939	91,488	91,753	95,190	339,477	370,370	377,753
Income before income taxes	35,618	28,669	29,391	13,241	42,926	26,613	22,169	23,809	106,919	115,517	87,581
Net income	$ 28,501	$ 23,353	$ 23,948	$ 10,800	$ 34,014	$ 21,386	$ 17,890	$ 19,400	$ 86,602	$ 92,690	$ 71,632
Earnings per share:
Basic	$ 1.48	$ 1.21	$ 1.24	$ 0.56	$ 1.77	$ 1.11	$ 0.93	$ 1.01	$ 4.49	$ 4.82	$ 3.73
Diluted	$ 1.47	$ 1.21	$ 1.24	$ 0.56	$ 1.76	$ 1.11	$ 0.93	$ 1.01	$ 4.47	$ 4.80	$ 3.71
Comprehensive income	$ 29,608	$ 24,087	$ 29,431	$ 12,284	$ 33,610	$ 23,862	$ 23,068	$ 25,193	$ 95,410	$ 105,733	$ 67,389
Extraordinary cash dividends declared per common share	$ 3.50				$ 3.50				$ 3.50	$ 3.50	$ 3.50
Cash dividends declared per common share	$ 0.27	$ 0.27	$ 0.27	$ 0.27	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 1.08	$ 1.00	$ 0.88