SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue Recognition Narrative (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		6 Months Ended
	May 31, 2018	Mar. 31, 2015	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue recognized			$ 607	$ 2,573	$ 1,653	$ 6,065
Deferred revenue			142		$ 142		$ 1,795
Collaborative Arrangement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Non-refundable upfront fees	$ 15,600	$ 11,000
Development milestone payments		10,000
Commercialization milestone payments		$ 30,000
Expected performance period	1 year 5 months 12 days
Basis point reduction					1.50%
Revenue recognized			600	$ 2,600	$ 1,700	$ 6,100
Deferred revenue			$ 100		100		$ 1,800
Milestone payment received					$ 10,000