LICENSE AGREEMENTS AND COMMITMENTS - MSK License Agreement (Details) - MSK License Agreement $ in Thousands		6 Months Ended		12 Months Ended
	Aug. 20, 2015 USD ($) shares	Jun. 30, 2020 USD ($) agreement	Jun. 30, 2019 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2019 USD ($)
Agreements
Number of license agreements | agreement		3
Issuance of common stock to strategic partner (in shares) | shares	1,428,500
Annual minimum royalties payable from fifth anniversary	$ 80
Expensed minimum royalty payments				$ 1,200
Clinical milestones potentially due	2,450
Regulatory milestones potentially due	9,000
Sales-based milestones potentially due	$ 20,000
Milestone expenses		$ 0	$ 0
Accrued short-term liabilities
Agreements
Short-term accrued minimum royalties		69			$ 29
Accrued milestone payments, current		225	150
Accrued long-term liabilities
Agreements
Long-term accrued minimum royalties		1,131			$ 1,171
Accrued milestone payments, noncurrent		$ 300	$ 300