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STOCK-BASED COMPENSATION - Summary of stock options (Details) - Stock option plans - USD ($)
$ / shares in Units, $ in Thousands
9 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2016
STOCK-BASED COMPENSATION        
Number of shares underlying options exercisable 134,723      
Number of shares underlying options unvested 100,199      
Number of shares underlying options outstanding 234,922      
Weighted-average exercise price per share exercisable $ 14.54      
Weighted-average exercise price per share unvested 19.53      
Weighted-average exercise price per share outstanding $ 16.67 $ 16.00 $ 14.93 $ 12.80
Aggregate intrinsic value exercisable $ 2,829      
Aggregate intrinsic value unvested 1,605      
Aggregate intrinsic value outstanding $ 4,433      
Weighted-average contractual term exercisable (years) 5 years 6 months      
Weighted-average contractual term unvested (years) 8 years      
Weighted-average contractual term outstanding (years) 6 years 7 months 6 days