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Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($)
$ / shares in Units, shares in Thousands, $ 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
Earnings Loss Per Share [Line Items]                      
Net income $ (26,224) $ 17,924 $ 11,076 $ 14,867 $ 31,124 $ 26,628 $ 18,884 $ 14,272 $ 17,643 $ 90,908 $ 105,931
Total weighted average common stock shares outstanding                 11,444 11,425 11,355
Employee stock options                 103 91 184
Adjusted weighted average shares                 11,547 11,516 11,539
Earnings per share:                      
Basic earnings per share $ (2.29) [1] $ 1.57 [1] $ 0.97 [1] $ 1.30 [1] $ 2.72 [1] $ 2.33 [1] $ 1.65 [1] $ 1.25 [1] $ 1.54 [1],[2] $ 7.96 [1],[2] $ 9.33 [2]
Diluted earnings per share $ (2.29) [1] $ 1.56 [1] $ 0.96 [1] $ 1.28 [1] $ 2.69 [1] $ 2.31 [1] $ 1.64 [1] $ 1.24 [1] $ 1.53 [1],[2] $ 7.89 [1],[2] $ 9.18 [2]
Class A                      
Earnings Loss Per Share [Line Items]                      
Net income                 $ 0 $ 0 $ 0
Total weighted average common stock shares outstanding                 9,409 9,390 9,320
Class B Convertible                      
Earnings Loss Per Share [Line Items]                      
Net income                 $ 0 $ 0 $ 0
Total weighted average common stock shares outstanding                 2,035 2,035 2,035
[1] The sum of the basic and diluted (loss) earnings per share for the four quarters may differ from annual earnings per share as the weighted-average shares outstanding are computed independently for each of the quarters presented.
[2] Basic and fully diluted earnings per share for class A and class B common stock are the same.