NET REVENUES FROM COLLABORATIONS - Schedule of Transaction Price Allocated (Details) - Global Strategic Collaboration - Regeneron $ in Thousands		3 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2020 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction Price Allocated	$ 555,100	$ 521,600
Research Services Obligation
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Standalone Selling Price	130,700	130,700
Transaction Price Allocated	200,600	183,100
C5 License Obligation
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Standalone Selling Price	97,600	97,600
Transaction Price Allocated	108,500	92,500
C5 Co-Co Obligation
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Standalone Selling Price	364,600	364,600
Transaction Price Allocated	$ 246,000	$ 246,000