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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Fair Value Measurement (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Fair value measurements    
Level 1 to Level 2 Transfers $ 0  
Level 2 to Level 1 Transfers 0  
Transfers into Level 3 0  
Transfers out of Level 3 0  
Recurring    
Fair value measurements    
Cash equivalents 173,587 $ 197,879
Recurring | Money market funds    
Fair value measurements    
Cash equivalents 173,587 197,879
Recurring | Level 2    
Fair value measurements    
Cash equivalents 173,587 197,879
Recurring | Level 2 | Money market funds    
Fair value measurements    
Cash equivalents $ 173,587 $ 197,879