SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue Recognition Narrative (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
	May 31, 2018	Mar. 31, 2015	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue recognized			$ 132	$ 142	$ 300	$ 1,795
Royalty revenue
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue recognized			132	0	300	$ 0
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
Basis point reduction						1.50%
Revenue recognized				100		$ 1,800
Milestone payment received					10,000
Collaborative Arrangement | Royalty revenue
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue recognized			$ 100	$ 0	$ 300	$ 0