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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, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]                          
Net premiums earned $ 87,377 $ 85,118 $ 82,749 $ 90,912 $ 92,097 $ 89,918 $ 90,728 $ 95,961 $ 346,156 $ 368,704 $ 375,894    
Net investment income 7,312 7,788 7,471 6,710 7,855 8,006 6,201 6,044 29,281 28,106 27,902    
Net realized gains (losses) on investments 114 (192) (388) (181) (1,468) 181 545 248 (647) (494) (2,494)    
Total revenues 94,937 92,804 89,925 97,542 98,558 98,206 97,563 102,335 375,208 396,662 400,954    
Income before income taxes 17,800 23,556 22,151 18,733 27,784 26,027 24,615 34,395 82,240 112,821 100,967    
Net income 649 16,577 15,481 13,524 19,073 17,896 16,639 24,257 46,231 77,865 70,462    
Net income available to common shareholders $ 649 $ 16,577 $ 15,481 $ 13,524 $ 19,073 $ 17,896 $ 16,639 $ 24,257 $ 46,231 $ 77,865 $ 70,462    
Earnings per share:                          
Basic $ 0.03 $ 0.86 $ 0.81 $ 0.71 $ 1.00 $ 0.94 $ 0.87 $ 1.27 $ 2.41 $ 4.08 $ 3.72    
Diluted $ 0.03 $ 0.86 $ 0.81 $ 0.70 $ 0.99 $ 0.93 $ 0.87 $ 1.27 $ 2.40 $ 4.05 $ 3.69    
Comprehensive income $ 1,228 $ 17,194 $ 17,437 $ 14,476 $ 11,985 $ 16,183 $ 19,568 $ 27,051 $ 50,335 $ 74,786 $ 70,239    
Extraordinary cash dividends declared per common share $ 3.50       $ 3.25       $ 3.50 $ 3.25 $ 3.00 $ 1.50 $ 0
Cash dividends declared per common share $ 0.20 $ 0.20 $ 0.20 $ 0.20 $ 0.18 $ 0.18 $ 0.18 $ 0.18 $ 0.80 $ 0.72 $ 0.60