Collaboration and License Agreements - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 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			$ 10,000,000.0		$ 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
Change in variable consideration constrained			$ 0
Revenues	$ 2,300,000	$ 2,000,000.0	4,900,000	$ 2,800,000
Deferred Revenue	1,700,000		1,700,000		3,800,000
Accounts Receivable, Related Parties	0		0		2,000,000.0
Deferred revenue current and non-current	4,000,000.0		4,000,000.0		7,900,000
Reimbursement Payment Received			2,700,000		1,700,000
Amount received on achievement of milestone	2,000,000.0		2,000,000.0
Unbilled accounts receivable related to reimbursable research and development cost	600,000		600,000		500,000
Acceleron Collaboration Agreement | Research Milestones
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate amount receivable on achievement of milestone	2,000,000.0		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		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		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		217,500,000
MyoKardia Collaboration and License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non refundable upfront payment received			10,000,000.0
Revenues	2,000,000.0	$ 0	4,200,000	$ 0
Deferred revenue current and non-current	5,900,000		5,900,000		10,000,000.0
Unbilled accounts receivable related to reimbursable research and development cost	100,000		100,000		$ 0
Upfront payment							$ 10,000,000.0
Payment for prepaid research funding							2,500,000
Prepaid research funding payment received	$ 2,500,000		$ 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