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STOCKHOLDERS' EQUITY - Stock option activity (Details) - Employee Stock Option [Member] - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2020
Dec. 31, 2019
Equity Incentive Plans Stock Option Activity [Line Items]    
Number of shares, Outstanding Beginning Balance 2,605,730  
Number of shares, Granted. 0  
Number of shares, Exercised (19,750)  
Number of shares, Forfeited 0  
Number of shares, Expired 0  
Number of shares, Outstanding Ending Balance 2,585,980 2,605,730
Weighted - average exercise price, Outstanding Beginning Balance $ 6.73  
Weighted - average exercise price, Granted 0  
Weighted - average exercise price, Exercised 4.10  
Weighted - average exercise price, Forfeited 0  
Weighted - average exercise price, Expired 0  
Weighted - average exercise price, Outstanding Ending Balance $ 6.75 $ 6.73
Weighted - average Contractual Term 8 years 8 months 23 days 8 years 11 months 1 day
Aggregate Intrinsic Value, Outstanding $ 9,153,390 $ 11,706,110