COLLABORATIVE RESEARCH AND LICENSE AGREEMENTS - Alexion Collaboration and Equity Agreements (Details)		1 Months Ended
	Oct. 22, 2018 USD ($) target milestone candidate $ / shares shares	Dec. 31, 2019 USD ($) target	Sep. 30, 2020 USD ($)
Disaggregation of Revenue [Line Items]
Deferred revenue		$ 394,988,000	$ 468,543,000
Alexion Agreements
Disaggregation of Revenue [Line Items]
Deferred revenue			52,300,000
Alexion Agreement
Disaggregation of Revenue [Line Items]
Number of treatment candidates | candidate	2
Initial non-creditable upfront payment	$ 22,000,000.0
Collaborative arrangement option exercise fee for each of candidates selected	$ 10,000,000.0
Collaborative arrangement term	10 years
Transaction price allocated to revenue arrangement	$ 5,900,000
Number of pathway targets | target	2
Transaction price	$ 45,200,000
Aggregate contingent milestone payments	14,500,000
Variable consideration	2,800,000
Variable consideration beyond initial milestones	$ 0
Number of initial research program milestones | milestone	3
Deferred revenue		$ 32,996,000	21,526,000
Alexion Agreement | Maximum
Disaggregation of Revenue [Line Items]
Additional payment receivable	$ 600,000,000.0
Option exercise fee	20,000,000.0
Development milestones receivable for each product	105,000,000.0
Aggregate sales milestones receivable	$ 160,000,000.0
Alexion Share Issuance Agreement
Disaggregation of Revenue [Line Items]
Number of shares issuable (in shares) | shares	835,834
Price per share (in dollars per share) | $ / shares	$ 17.95
Expected proceeds from issuance of common stock	$ 15,000,000.0
Share purchase price allocated to equity	9,100,000
Transaction price allocated to revenue arrangement	5,900,000
Alexion Amendment
Disaggregation of Revenue [Line Items]
Option exercise fee	20,000,000.0
Collaborative arrangement option exercise fee for each of candidates selected	$ 10,000,000.0
Number of pathway targets | target	2	4
Transaction price	$ 35,000,000.0
Aggregate contingent milestone payments	$ 15,000,000.0
Number of additional pathway targets | target	2
Deferred revenue		$ 20,000,000	$ 25,808,000