NET REVENUES FROM COLLABORATIONS - Schedule of Revenue Recognized by Accounting Guidance (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized under ASC 606	$ 177,566	$ 99,476
Regeneron Pharmaceuticals | Global Strategic Collaboration
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	25,800	16,500
Regeneron Pharmaceuticals | Research Services Obligation | Global Strategic Collaboration
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized under ASC 606	9,800	12,300
Regeneron Pharmaceuticals | C5 License Obligation | Global Strategic Collaboration
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized under ASC 606	13,500	0
Regeneron Pharmaceuticals | C5 Co-Co Obligation | Global Strategic Collaboration
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized under ASC 808	$ 2,500	$ 4,200