Annual report pursuant to Section 13 and 15(d)

FAIR VALUE MEASUREMENTS - Assets Measured at Fair Value on Recurring Basis (Details)

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FAIR VALUE MEASUREMENTS - Assets Measured at Fair Value on Recurring Basis (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Cash equivalents    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Amounts included in cash and cash equivalents $ 85,330 $ 73,608
Fair Value, Measurements, Recurring | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value on a recurring basis 85,330 73,608
Fair Value, Measurements, Recurring | Level 1 | Cash equivalents    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Amounts included in cash and cash equivalents 85,330 73,608
Fair Value, Measurements, Recurring | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value on a recurring basis 0 0
Fair Value, Measurements, Recurring | Level 2 | Cash equivalents    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Amounts included in cash and cash equivalents 0 0
Fair Value, Measurements, Recurring | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value on a recurring basis 0 0
Fair Value, Measurements, Recurring | Level 3 | Cash equivalents    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Amounts included in cash and cash equivalents 0 0
Fair Value, Measurements, Recurring | Total    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets measured at fair value on a recurring basis 85,330 73,608
Fair Value, Measurements, Recurring | Total | Cash equivalents    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Amounts included in cash and cash equivalents $ 85,330 $ 73,608