STOCKHOLDERS' EQUITY - STOCK-BASED COMPENSATION EXPENSE FOR EMPLOYEES AND DIRECTORS (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Stock-Based Compensation Expense
Stock-based compensation expense included in operating expenses	$ 6,079	$ 6,368	$ 8,144
Research and development
Stock-Based Compensation Expense
Stock-based compensation expense included in operating expenses	1,640	949	988
General and administrative
Stock-Based Compensation Expense
Stock-based compensation expense included in operating expenses	$ 4,439	$ 5,419	$ 7,156

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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