Collaboration and License Agreements - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2020	Jul. 31, 2020
Acceleron Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non refundable upfront payment received	$ 2,000,000.0	$ 10,000,000.0
Range of tiered royalties on net sales	mid single-digit percentage to a low double-digit percentage
Revenues	$ 6,300,000
Cumulative Catch-Up Adjustment	500,000
Deferred revenue current and non-current	7,900,000	10,000,000.0
Reimbursement Payment Recieved	1,700,000
Unbilled Accounts Receivable Related To Reimbursable Research And Development Cost	500,000
Accounts and Financing Receivable, after Allowance for Credit Loss	2,000.0	0
Acceleron Collaboration Agreement | Research Milestones
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate amount receivable on achievement of milestone	2,000,000.0
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
Research Milestones
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenues	2,000,000.0	3,400,000
MyoKardia Collaboration and License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non refundable upfront payment received	10,000,000.0
Revenues	2,500,000	0
Deferred revenue current and non-current	10,000,000.0	$ 0
Unbilled Accounts Receivable Related To Reimbursable Research And Development Cost	$ 0
Upfront payment				$ 10,000,000.0
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				$ 150,000,000.0