PREPAID EXPENSES AND OTHER CURRENT ASSETS (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
PREPAID EXPENSES AND OTHER CURRENT ASSETS
Prepaid research and development expenses	$ 8,607	$ 5,872
Prepaid rent expenses	111	142
Prepaid insurance expenses	447	1,196
Interest Receivable	136	10
Other prepaid expenses and current assets	131	81
Total prepaid expenses & other current assets	$ 9,432	$ 7,301