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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Assets measured at fair value (Details) - Fair Value, Measurements, Recurring [Member] - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Quoted Prices in Active Markets for Identical Assets (Level 1)    
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES    
Total $ 46,174 $ 47,939
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds (included in cash and cash equivalents)    
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES    
Money market funds (included in cash and cash equivalents) 46,174 47,939
Significant Other Observable Inputs (Level 2)    
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES    
Total 0 0
Significant Other Observable Inputs (Level 2) | Money market funds (included in cash and cash equivalents)    
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES    
Money market funds (included in cash and cash equivalents) 0 0
Significant Unobservable Inputs (Level 3)    
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES    
Total 0 0
Significant Unobservable Inputs (Level 3) | Money market funds (included in cash and cash equivalents)    
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES    
Money market funds (included in cash and cash equivalents) $ 0 $ 0