STOCKHOLDERS' EQUITY - STOCK-BASED COMPENSATION EXPENSE FOR EMPLOYEES AND DIRECTORS (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock-Based Compensation Expense
Stock-based compensation expense included in operating expenses	$ 6,895	$ 6,079	$ 6,368
Research and development
Stock-Based Compensation Expense
Stock-based compensation expense included in operating expenses	2,337	1,640	949
General and administrative
Stock-Based Compensation Expense
Stock-based compensation expense included in operating expenses	$ 4,558	$ 4,439	$ 5,419

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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