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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
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Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract] |  |  |  |  |  |  |  |  |  |  |
Premiums earned | $ 95,034 | $ 90,504 | $ 95,569 | $ 94,787 | $ 97,070 | $ 95,928 | $ 93,516 | $ 89,233 | $ 375,894 | $ 375,747 | $ 329,983
Net investment income | 7,256 | 6,923 | 6,890 | 6,833 | 7,166 | 6,495 | 6,845 | 6,708 | 27,902 | 27,214 | 27,029
Net realized gains (losses) on investments | 24 | 40 | (2,617) | 59 | 516 | (152) | 232 | 101 | (2,494) | 697 | (1,211)
Total revenues | 101,760 | 97,470 | 99,936 | 101,788 | 104,886 | 102,336 | 100,624 | 96,173 | 400,954 | 404,019 | 356,335
Income before income taxes | 33,768 | 25,890 | 20,112 | 21,197 | 23,936 | 19,348 | 17,061 | 13,404 | 100,967 | 73,749 | 59,204
Net income | 23,073 | 17,940 | 14,319 | 15,130 | 16,865 | 13,479 | 12,773 | 10,549 | 70,462 | 53,666 | 43,637
Net income available to common shareholders | $ 23,073 | $ 17,940 | $ 14,319 | $ 15,130 | $ 16,865 | $ 13,479 | $ 12,773 | $ 10,549 | $ 70,462 | $ 53,666 | $ 43,495
Earnings per share: |  |  |  |  |  |  |  |  |  |  |
Basic | $ 1.21 | $ 0.95 | $ 0.76 | $ 0.80 | $ 0.90 | $ 0.72 | $ 0.69 | $ 0.57 | $ 3.72 | $ 2.88 | $ 2.37
Diluted | $ 1.21 | $ 0.94 | $ 0.75 | $ 0.79 | $ 0.89 | $ 0.71 | $ 0.68 | $ 0.56 | $ 3.69 | $ 2.84 | $ 2.32
Comprehensive income | $ 22,299 | $ 19,248 | $ 12,744 | $ 15,948 | $ 17,597 | $ 13,596 | $ 15,789 | $ 13,789 | $ 70,239 | $ 60,771 | $ 36,363
Extraordinary cash dividends declared per common share | $ 3.00 |  |  |  | $ 1.00 |  |  | $ 0.50 | $ 3.00 | $ 1.50 | $ 0.00
Cash dividends declared per common share | $ 0.15 | $ 0.15 | $ 0.15 | $ 0.15 | $ 0.12 | $ 0.12 | $ 0.12 | $ 0.12 | $ 0.60 | $ 0.48 | $ 0.32

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