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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Fair Value Measurement (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Fair value measurements    
Level 1 to Level 2 Transfers $ 0 $ 0
Level 2 to Level 1 Transfers 0 0
Transfers into Level 3 0 0
Transfers out of Level 3 0 0
Recurring    
Fair value measurements    
Cash equivalents 197,879  
Recurring | Money market funds    
Fair value measurements    
Cash equivalents 197,879 $ 0
Recurring | Level 2    
Fair value measurements    
Cash equivalents 197,879  
Recurring | Level 2 | Money market funds    
Fair value measurements    
Cash equivalents $ 197,879