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CREDIT RISK AND CONCENTRATION - Additional Information (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Concentration Risk        
Increase or decrease in interest rates of short-term investments that will not significantly affect the fair value of investment portfolio (as a percent)   10.00%    
Net sales [1]   $ 24,310 $ 65,623 $ 115,944
China        
Concentration Risk        
Net sales   2,286 2,439 $ 3,146
Cash, Cash Equivalents and Short Term Investments | Countries Other than US | Credit Concentration Risk        
Concentration Risk        
Cash and cash equivalents and short-term investments   $ 28,600 $ 35,400  
Accounts Receivable | Customer Concentration Risk | Customer A        
Concentration Risk        
Concentration of risk (as a percent)   1.00% 1.00%  
Accounts Receivable | Customer Concentration Risk | Customer B        
Concentration Risk        
Concentration of risk (as a percent)   91.00% 91.00%  
Net sales   $ 21,000    
Accounts Receivable | Customer Concentration Risk | Customer B | Subsequent Event        
Concentration Risk        
Net sales $ 16,000      
Subsidiaries | Cash, Cash Equivalents and Short Term Investments | China | Credit Concentration Risk        
Concentration Risk        
Cash and cash equivalents and short-term investments   $ 12,200 $ 3,100  
[1] Revenue recognized in the years ended December 31, 2020, 2019 and 2018 that was included in deferred revenue (including current and non-current) and customer advances as of the beginning of each respective period was $1.2 million, $2.8 million and $4.0 million, respectively.