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Quarterly Financial Data (Unaudited) - Unaudited Quarterly Financial Data (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]                            
Net premiums earned $ 88,837 $ 85,184 $ 88,995 $ 87,310 $ 87,377 $ 85,118 $ 82,749 $ 90,912 $ 350,326 $ 346,156 $ 368,704      
Net investment income 8,056 7,884 7,303 7,209 7,312 7,788 7,471 6,710 30,452 29,281 28,106      
Net realized gains (losses) on investments (65) (329) (1,111) (31) 114 (192) (388) (181) (1,536) (647) (494)      
Total revenues 94,669 93,529 95,380 94,175 94,937 92,804 89,925 97,542 377,753 375,208 396,662      
Income before income taxes 22,777 24,460 20,930 19,414 17,800 23,556 22,151 18,733 87,581 82,240 112,821      
Net income $ 18,806 $ 19,701 $ 16,956 $ 16,169 $ 649 $ 16,577 $ 15,481 $ 13,524 $ 71,632 $ 46,231 $ 77,865      
Earnings per share:                            
Basic $ 0.98 $ 1.03 $ 0.88 $ 0.84 $ 0.03 $ 0.86 $ 0.81 $ 0.71 $ 3.73 $ 2.41 $ 4.08      
Diluted $ 0.98 $ 1.02 $ 0.88 $ 0.84 $ 0.03 $ 0.86 $ 0.81 $ 0.70 $ 3.71 $ 2.40 $ 4.05      
Comprehensive income $ 22,176 $ 17,582 $ 17,109 $ 10,321 $ 1,228 $ 17,194 $ 17,437 $ 14,476 $ 67,389 $ 50,335 $ 74,786      
Extraordinary cash dividends declared per common share $ 3.50       $ 3.50       $ 3.50 $ 3.50 $ 3.25 $ 3.00 $ 1.50 $ 0
Cash dividends declared per common share $ 0.22 $ 0.22 $ 0.22 $ 0.22 $ 0.20 $ 0.20 $ 0.20 $ 0.20 $ 0.88 $ 0.80 $ 0.72