Significant Agreements - Celgene, Accounting Analysis (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Significant Agreements
Revenue from contract with customer	$ 22,561		$ 4,208	$ 66,656	$ 34,655
REBLOZYL Agreement | Collaboration, License, and Option Agreement
Significant Agreements
Revenue from contract with customer		$ 25,000
Payments received				234,300
Sotarcept | Collaboration, License, and Option Agreement
Significant Agreements
Payments received				44,900
Regulatory Milestone | Luspatercept | Collaboration, License, and Option Agreement
Significant Agreements
Collaborative arrangement potential milestone payments on acceptance of regulatory application	$ 20,000			$ 20,000