RESTRUCTURING PLAN - Narrative (Details) $ in Thousands	3 Months Ended		6 Months Ended	12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($) position
Restructuring and Related Activities [Abstract]
Expected position eliminations | position				44
Expected position eliminations, percent				50.00%
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 900	$ 1,100		$ 2,034
Research and development
Restructuring Cost and Reserve [Line Items]
Restructuring charges	800	900		1,700
General and administrative
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 100	$ 200		$ 300
Forecast
Restructuring Cost and Reserve [Line Items]
Restructuring charges			$ 200