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FAIR VALUE MEASUREMENTS - Embedded derivative liabilities (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Beginning balance at January 1 $ 95,342    
Change in fair value 66,397    
Ending balance at December 31 161,486 $ 95,342  
Embedded derivatives      
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Beginning balance at January 1 $ 0 786  
Issuance of derivative   382 $ 2,673
Change in fair value   (1,168) (1,887)
Ending balance at December 31   $ 0 $ 786