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Liquidity and Capital Resources (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Dec. 31, 2019
Liquidity and Capital Resources [Abstract]        
Accumulated Deficit $ (345,158)   $ (337,406)  
Stockholders' equity 26,178 $ 21,774 $ 34,170 $ 20,320
Cash, cash equivalents and investments available-for-sale 46,100      
Working capital 48,000      
Net cash used in operating activities $ (12,495) $ (6,538)