FAIR VALUE OF MEASUREMENTS AND INVESTMENT SECURITIES - Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Restricted cash	$ 1,501	$ 1,501
Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total cash equivalents	12,481	27,463
Level 1 | Money market funds and treasury bills
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total cash equivalents	12,481	27,463
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