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Summary of Option Activity (Detail) - Incentive Stock Options - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Incentive Stock Option Plans, Beginning balance 472,783 541,905 626,845
Incentive Stock Option Plans, Option exercised (88,719) (55,979) (58,900)
Incentive Stock Option Plans, Option forfeited   (13,143) (26,040)
Incentive Stock Option Plans, Ending balance 384,064 472,783 541,905
Weighted Average Price Per Share, Beginning balance $ 9.38 $ 9.33 $ 9.25
Weighted Average Price Per Share, Option exercised 10.62 8.37 7.50
Weighted Average Price Per Share, Option forfeited   11.49 11.49
Weighted Average Price Per Share, Ending balance 9.10 9.38 9.33
Exercisable Weighted Average Price Per Share, Beginning balance 9.38 7.97 7.73
Exercisable Weighted Average Price Per Share, Ending balance $ 9.10 $ 9.38 $ 7.97