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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - STOCK BASED COMPENSATION (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Stock-Based Compensation Expense        
Stock-based compensation expense included in operating expenses $ 1,434 $ 1,545 $ 2,860 $ 3,159
Research and development        
Stock-Based Compensation Expense        
Stock-based compensation expense included in operating expenses 413 266 653 421
General and administrative        
Stock-Based Compensation Expense        
Stock-based compensation expense included in operating expenses $ 1,021 $ 1,279 $ 2,207 $ 2,738