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SHARE INCENTIVE PLAN (Details)
12 Months Ended
Apr. 02, 2018
$ / shares
shares
Jul. 01, 2017
$ / shares
shares
Jun. 22, 2017
item
$ / shares
shares
Dec. 31, 2018
$ / shares
shares
Dec. 31, 2017
$ / shares
shares
Dec. 31, 2016
$ / shares
SHARE INCENTIVE PLAN            
Ordinary shares, par value (in dollars per share) | $ / shares       $ 0.001 $ 0.001 $ 0.001
Share options granted (in shares) | shares       20,000 2,109,305  
Exercise price per share of the options granted | $ / shares       $ 0.01 $ 11.42  
Directors            
SHARE INCENTIVE PLAN            
Share options granted (in shares) | shares     1,286,878      
Exercise price per share of the options granted | $ / shares     $ 11.66      
Employees            
SHARE INCENTIVE PLAN            
Share options granted (in shares) | shares 20,000          
Exercise price per share of the options granted | $ / shares $ 0.01          
Vesting of share-based compensation (as a percent) 25.00%          
Shares vesting quarterly in twelve quarters (as a percent) 75.00%          
Number of quarters of vesting 12          
Director and consultant            
SHARE INCENTIVE PLAN            
Share options granted (in shares) | shares   50,300        
Exercise price per share of the options granted | $ / shares   $ 1.48        
Completion of IPO before June 22, 2018 | Directors            
SHARE INCENTIVE PLAN            
Vesting of share-based compensation (as a percent)     25.00%      
Shares vesting quarterly in twelve quarters (as a percent)     75.00%      
Number of quarters of vesting | item     12      
Non completion of IPO before June 22, 2018 | Directors            
SHARE INCENTIVE PLAN            
Vesting of share-based compensation (as a percent)     25.00%      
Shares vesting quarterly in twelve quarters (as a percent)     75.00%      
Number of quarters of vesting | item     12      
Completion of IPO on September 27, 2018 | Employees            
SHARE INCENTIVE PLAN            
Share options granted (in shares) | shares     772,127      
Exercise price per share of the options granted | $ / shares     $ 11.66      
Vesting of share-based compensation (as a percent)     25.00%      
Shares vesting quarterly in twelve quarters (as a percent)     75.00%      
Number of quarters of vesting | item     12      
2009 Share Incentive Plan            
SHARE INCENTIVE PLAN            
Total number shares authorized | shares       2,573,756    
Ordinary shares, par value (in dollars per share) | $ / shares       $ 0.001    
2017 Share Incentive Plan            
SHARE INCENTIVE PLAN            
Total number shares authorized | shares       2,059,005    
Additional annual increase in number of shares (as a percent)       2.00%