SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets:
U.S. treasury securities (included in short-term investments)	$ 2,000	$ 29,900
Level 1
Assets:
Money market funds (included in cash and cash equivalents)	25,263	32,936
U.S. treasury securities (included in short-term investments)	1,999	29,875
Total assets	27,262	62,811
Liabilities:
Common Stock warrants (included in accrued and other liabilities)	0	0
Level 3
Assets:
Money market funds (included in cash and cash equivalents)	0	0
U.S. treasury securities (included in short-term investments)	0	0
Total assets	0	0
Liabilities:
Common Stock warrants (included in accrued and other liabilities)	$ 100	$ 82