FAIR VALUE OF MEASUREMENTS AND INVESTMENT SECURITIES - Assets Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Restricted cash, non-current	$ 1,500	$ 0
Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total cash equivalents	55,253	77,170
Level 1 | Money market funds and treasury bills
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total cash equivalents	55,253	77,170
Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total cash equivalents	0	0
Level 2 | Money market funds and treasury bills
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total cash equivalents	0	0
Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total cash equivalents	0	0
Level 3 | Money market funds and treasury bills
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total cash equivalents	$ 0	$ 0