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STOCK AWARD PLAN AND STOCK-BASED COMPENSATION - Stock Option Activity for Employees and Non-Employees (Details) - Employees and Non-Employees Stock Option - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Shares Issuable Pursuant to Stock Options    
Outstanding at the beginning of period (in shares) 8,498,047  
Exercised (in shares) (87,500)  
Forfeited (in shares) (31,875)  
Outstanding at the end of the period (in shares) 8,378,672 8,498,047
Exercisable at the end of the period (in shares) 4,078,271  
Available for future grant (in shares) 801,654  
Weighted-Average Exercise Price    
Outstanding at the beginning of period (in dollars per share) $ 6.99  
Exercised (in dollars per share) 6.00  
Forfeited (in dollars per share) 6.51  
Outstanding at the end of the period (in dollars per share) 7.00 $ 6.99
Exercisable at the end of the period (in dollars per share) $ 6.46  
Weighted-Average Remaining Contractual Term Outstanding (Years) 7 years 9 months 18 days 8 years 1 month 6 days
Weighted-Average Remaining Contractual Term Exercisable (Years) 6 years 9 months 18 days  
Total Intrinsic Value Outstanding at the begining of period $ 5,214  
Total Intrinsic Value Outstanding at the end of period 11,212 $ 5,214
Total Intrinsic Value Exercisable $ 7,635