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SIGNIFICANT ACCOUNTING POLICIES - Marketable Securities (Details) - USD ($)
$ in Thousands
Jun. 30, 2021
Dec. 31, 2020
Marketable Securities [Line Items]    
Marketable securities $ 144,931 $ 25,736
U.S. Treasury securities    
Marketable Securities [Line Items]    
Marketable securities 76,590 25,736
Commercial paper (A-1 or P-1 rated)    
Marketable Securities [Line Items]    
Marketable securities $ 68,341 $ 0