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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, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]                        
Net premiums earned $ 92,097 $ 89,918 $ 90,728 $ 95,961 $ 95,034 $ 90,504 $ 95,569 $ 94,787 $ 368,704 $ 375,894 $ 375,747  
Net investment income 7,855 8,006 6,201 6,044 7,256 6,923 6,890 6,833 28,106 27,902 27,214  
Net realized gains (losses) on investments (1,468) 181 545 248 24 40 (2,617) 59 (494) (2,494) 697  
Total revenues 98,558 98,206 97,563 102,335 101,760 97,470 99,936 101,788 396,662 400,954 404,019  
Income before income taxes 27,784 26,027 24,615 34,395 33,768 25,890 20,112 21,197 112,821 100,967 73,749  
Net income 19,073 17,896 16,639 24,257 23,073 17,940 14,319 15,130 77,865 70,462 53,666  
Net income available to common shareholders $ 19,073 $ 17,896 $ 16,639 $ 24,257 $ 23,073 $ 17,940 $ 14,319 $ 15,130 $ 77,865 $ 70,462 $ 53,666  
Earnings per share:                        
Basic $ 1.00 $ 0.94 $ 0.87 $ 1.27 $ 1.21 $ 0.95 $ 0.76 $ 0.80 $ 4.08 $ 3.72 $ 2.88  
Diluted $ 0.99 $ 0.93 $ 0.87 $ 1.27 $ 1.21 $ 0.94 $ 0.75 $ 0.79 $ 4.05 $ 3.69 $ 2.84  
Comprehensive income $ 11,985 $ 16,183 $ 19,568 $ 27,051 $ 22,299 $ 19,248 $ 12,744 $ 15,948 $ 74,786 $ 70,239 $ 60,771  
Extraordinary cash dividends declared per common share $ 3.25       $ 3.00       $ 3.25 $ 3.00 $ 1.50 $ 0
Cash dividends declared per common share $ 0.18 $ 0.18 $ 0.18 $ 0.18 $ 0.15 $ 0.15 $ 0.15 $ 0.15 $ 0.72 $ 0.60 $ 0.48