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STOCKHOLDERS' EQUITY - STOCK-BASED COMPENSATION EXPENSE FOR EMPLOYEES AND DIRECTORS (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stock-Based Compensation Expense      
Stock-based compensation expense included in operating expenses $ 8,144 $ 8,245 $ 8,397
Research and development      
Stock-Based Compensation Expense      
Stock-based compensation expense included in operating expenses 988 1,275 2,139
Restructuring charges      
Stock-Based Compensation Expense      
Stock-based compensation expense included in operating expenses     307
General and administrative      
Stock-Based Compensation Expense      
Stock-based compensation expense included in operating expenses $ 7,156 $ 6,970 $ 5,951