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, 2018	Dec. 31, 2017	Oct. 31, 2017	Mar. 31, 2015	Sep. 30, 2019	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Jan. 01, 2018
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue recognized						$ 1,183	$ 3,042		$ 7,248	$ 8,415
Deferred revenue, current portion						2,464			2,464		$ 8,117
General and administrative						3,901	$ 1,206		$ 7,290	$ 4,694
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			$ 5,000			1,200		$ 5,000	$ 7,200
Deferred revenue, current portion						$ 2,500			$ 2,500
Prepayment option percent				50.00%
Prepayment amount			$ 5,000
Milestone payment received		$ 5,000
Accounting Standards Update 2014-09
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Effect of adoption of Topic 606												$ 2,734
Sublicensing Costs | Collaborative Arrangement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
General and administrative				$ 1,000				$ 1,000