Collaboration and License Agreements - Additional Information (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Jul. 31, 2020
Acceleron Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non refundable upfront payment received	$ 10,000,000		$ 2,000,000	$ 10,000,000
Range of tiered royalties on net sales	mid single-digit percentage to a low double-digit percentage
Change in variable consideration constrained	$ 0
Revenues	2,600,000	$ 800,000
Deferred Revenue			2,100,000
Accounts Receivable, Related Parties	0
Deferred revenue current and non-current	5,800,000	7,900,000
Reimbursement Payment Received	2,200,000	1,700,000
Amount received on achievement of milestone	2,000,000
Unbilled accounts receivable related to reimbursable research and development cost			500,000
Acceleron Collaboration Agreement | Research Milestones
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate amount receivable on achievement of milestone	2,000,000
Acceleron Collaboration Agreement | Research Milestones | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate amount receivable on achievement of milestone	18,500,000
Acceleron Collaboration Agreement | Clinical and Regulatory Milestones | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate amount receivable on achievement of milestone	202,500,000
Acceleron Collaboration Agreement | Worldwide Net Sales Milestones | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate amount receivable on achievement of milestone	217,500,000
MyoKardia Collaboration and License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non refundable upfront payment received	10,000,000
Revenues	2,200,000	$ 0
Deferred revenue current and non-current	7,800,000		$ 10,000,000
Unbilled accounts receivable related to reimbursable research and development cost	0
Upfront payment					$ 10,000,000
Payment for prepaid research funding					2,500,000
Prepaid research funding payment received	$ 2,500,000
MyoKardia Collaboration and License Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment for certain identified targets					298,500,000
Milestone payment for certain other identified targets					$ 150,000,000