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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Assets measured at fair value on recurring basis (Details) - Fair value on recurring basis - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Fair Value Measurement, Quoted Prices in Active Markets for Identical Assets (Level 1)    
Schedule of Available-for-sale Securities [Line Items]    
Total assets measured at fair value $ 50,790 $ 43,896
Fair Value Measurement, Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Corporate debt securities 951 869
Fair Value Measurement, Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds (included in cash and cash equivalents)    
Schedule of Available-for-sale Securities [Line Items]    
Money market funds (included in cash and cash equivalents) 49,839 43,027
Fair Value Measurement, Significant Other Observable Inputs (Level 2)    
Schedule of Available-for-sale Securities [Line Items]    
Total assets measured at fair value
Fair Value Measurement, Significant Other Observable Inputs (Level 2) | Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Corporate debt securities
Fair Value Measurement, Significant Unobservable Inputs (Level 3)    
Schedule of Available-for-sale Securities [Line Items]    
Total assets measured at fair value
Fair Value Measurement, Significant Unobservable Inputs (Level 3) | Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Corporate debt securities