SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Assets measured at fair value on recurring basis (Details) - Fair value on recurring basis - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule of Available-for-sale Securities [Line Items]
Total assets measured at fair value	$ 47,939	$ 49,986
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)	47,939	49,986
Significant Other Observable Inputs (Level 2)
Schedule of Available-for-sale Securities [Line Items]
Total assets measured at fair value	0	0
Significant Other Observable Inputs (Level 2) | 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)		0
Significant Unobservable Inputs (Level 3)
Schedule of Available-for-sale Securities [Line Items]
Total assets measured at fair value	$ 0	0
Significant Unobservable Inputs (Level 3) | 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)		$ 0