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Stock-Based Compensation - Summary of Activity Under Stock Option Plan (Detail)
$ / shares in Units, $ in Thousands
 6 Months Ended
Jun. 30, 2018
USD ($)
$ / shares
shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]  
Outstanding at beginning period - Options | shares 1,273,625
Options granted | shares 34,500
Options exercised | shares (99,950)
Options canceled, forfeited, or expired | shares (5,000)
Outstanding at ending period - Options | shares 1,203,175
Exercisable at ending period - Options | shares 542,925
Outstanding at beginning period - Wgtd. Avg. Exercise Price | $ / shares $ 25.90
Options granted - Wgtd. Avg. Exercise Price | $ / shares 54.68
Options exercised - Wgtd. Avg. Exercise Price | $ / shares 16.45
Options canceled, forfeited, or expired - Wgtd. Avg. Exercise Price | $ / shares 48.02
Outstanding at ending period - Wgtd. Avg. Exercise Price | $ / shares 27.41
Exercisable at ending period - Wgtd. Avg. Exercise Price | $ / shares $ 21.17
Outstanding at ending period - Wgtd. Avg. Remaining Contractual Term, years 9 years 9 months 29 days
Exercisable at ending period - Wgtd. Avg. Remaining Contractual Term, years 7 years 3 months 25 days
Outstanding at ending period - Aggregate Intrinsic Value | $ $ 38,244
Exercisable at ending period - Aggregate Intrinsic Value | $ $ 20,648