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SHARE INCENTIVE PLAN - Fair value of the options granted, assumptions used (Details)
12 Months Ended
Apr. 02, 2018
multiple
Dec. 31, 2018
$ / shares
Dec. 31, 2017
multiple
$ / shares
Assumptions used      
Risk-free interest rate   2.73%  
Expected volatility   38.00% 40.00%
Exercise multiples 2.2    
Fair value of underlying ordinary share | $ / shares   $ 17.40 $ 11.67
Minimum      
Assumptions used      
Risk-free interest rate     2.15%
Exercise multiples     2.2
Maximum      
Assumptions used      
Risk-free interest rate     2.31%
Exercise multiples     2.8