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SHARE INCENTIVE PLAN - Share option activities (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Number of options outstanding        
Outstanding at beginning of year 4,079,261 2,536,956 2,566,256  
Granted 20,000 2,109,305    
Exercised   (550,000)    
Forfeited (42,250) (17,000) (29,300)  
Outstanding at end of year 4,057,011 4,079,261 2,536,956 2,566,256
Exercisable at end of year 3,517,781      
Weighted average exercise price        
Outstanding at beginning of year $ 7.09 $ 2.15 $ 2.15  
Granted 0.01 11.42    
Exercised   1.08    
Forfeited 11.66 2.32 1.76  
Outstanding at end of year 7.00 7.09 2.15 $ 2.15
Exercisable at end of the year 6.36      
Weighted average grant-date fair value per option        
Outstanding at beginning of year 3.57 1.03 1.02  
Granted 17.39 5.75    
Exercised   0.22    
Forfeited 5.34 0.79 0.41  
Outstanding at end of year 3.62 $ 3.57 $ 1.03 $ 1.02
Exercisable at end of the year $ 3.09      
Weighted-average remaining contractual term (years)        
Weighted-average remaining contractual term (in years) 5 years 11 months 5 days 6 years 10 months 28 days 4 years 6 months 11 days 5 years 6 months 7 days
Weighted-average remaining contractual term, granted (in years) 9 years 3 months 4 days      
Weighted-average remaining contractual term, exercisable at end of the year (in years) 5 years 6 months 11 days      
Aggregate intrinsic value        
Aggregate intrinsic value $ 10,337 $ 51,117 $ 15,281 $ 4,639
Aggregate intrinsic value, granted 117      
Aggregate intrinsic value, exercisable at end of the year $ 10,220