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Stock Incentive Plans - Summary of Stock Options Activity (Detail) - Stock Options [Member]
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Outstanding beginning balance, Number of Options | shares 303,200
Options exercised, Number of Options | shares (10,800)
Outstanding ending balance, Number of Options | shares 292,400
Number of Options, exercisable stock options | shares 228,650
Outstanding beginning balance, Weighted Average Exercise Price | $ / shares $ 17.89
Options exercised, Weighted Average Exercise Price | $ / shares 15.08
Outstanding ending balance, Weighted Average Exercise Price | $ / shares 17.99
Weighted Average Exercise Price, exercisable stock options | $ / shares $ 16.46
Weighted Average Remaining Contractual Term of outstanding stock options 3 years 6 months 25 days
Weighted Average Remaining Contractual Term of exercisable stock options 3 years 1 month 24 days
Aggregate Intrinsic Value, outstanding stock option | $ $ 13,596
Aggregate Intrinsic Value, exercisable stock option | $ $ 10,982