NET REVENUES FROM COLLABORATIONS - Schedule of Deferred Revenue (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Apr. 08, 2019
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 127,207	$ 77,821
Global Strategic Collaboration | Regeneron Pharmaceuticals
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction Price Allocated	531,800	531,800	$ 521,600
Deferred revenue	345,000	393,600
Research Services Obligation | Global Strategic Collaboration | Regeneron Pharmaceuticals
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction Price Allocated		200,600	183,100
Deferred revenue	54,900	84,800
C5 License Obligation | Global Strategic Collaboration | Regeneron Pharmaceuticals
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction Price Allocated		85,200	92,500
Deferred revenue	58,700	65,800
C5 Co-Co Obligation | Global Strategic Collaboration | Regeneron Pharmaceuticals
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction Price Allocated		246,000	$ 246,000
Deferred revenue	$ 231,400	$ 243,000