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STOCK-BASED COMPENSATION - Fair value (Details) - Restricted shares of common stock - USD ($)
9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Stock-based compensation    
Fair value, shares vested $ 3,174,000 $ 4,431,000
Chairman of Board and Director    
Stock-based compensation    
Fair value, shares vested   $ 1,100,000