COLLABORATIVE RESEARCH AND LICENSE AGREEMENTS - Alexion Summary of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue	$ 48,875	$ 8,035	$ 123,351	$ 16,824
Total
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue	13,639	1,451	23,464	2,954
Alexion Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue	11,358	1,451	19,273	2,954
Alexion Amendment
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue	$ 2,281	$ 0	$ 4,191	$ 0