Significant Agreements - Celgene, Accounting Analysis (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Significant Agreements
Revenue from contract with customer	$ 4,344	$ 2,780
Luspatercept \| Collaboration, License, and Option Agreement
Significant Agreements
Payments received	186,200
Sotarcept \| Collaboration, License, and Option Agreement
Significant Agreements
Payments received	44,800
Cost-sharing, net
Significant Agreements
Revenue from contract with customer	2,824	2,780
Cost-sharing, net \| Collaboration, License, and Option Agreement
Significant Agreements
Revenue from contract with customer	4,300	$ 2,800
Regulatory Milestone \| Luspatercept \| Collaboration, License, and Option Agreement
Significant Agreements
Collaborative arrangement potential milestone payments on acceptance of regulatory application	$ 25,000

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