Collaboration, License and Research Agreements - Additional Information (Details)			1 Months Ended		3 Months Ended			9 Months Ended
	Jun. 16, 2020 shares	Nov. 30, 2019 shares	Jun. 30, 2020 shares	Apr. 30, 2019 USD ($) Product	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 shares	Sep. 30, 2021 USD ($) Obligation	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized					$ 2,163,000	$ 1,746,000		$ 7,474,000	$ 4,645,000
Common Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Outstanding shares of convertible preferred stock converted into common stock | shares							17,921,000
Initial Public Offering
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Outstanding shares of convertible preferred stock converted into common stock | shares	17,921,069
Initial Public Offering | Common Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Outstanding shares of convertible preferred stock converted into common stock | shares			17,921,069
Lilly Note
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from issuance of convertible notes				$ 15,000,000.0
Lilly Note | Initial Public Offering | Common Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Outstanding shares of convertible preferred stock converted into common stock | shares			2,169,396
Lilly Note | Series C Convertible Preferred Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Conversion of convertible note payable to preferred stock | shares		4,576,342
Research Collaboration and License Agreement with Eli Lilly
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront fees received				$ 20,000,000.0
License agreement expiry period				10 years
Number of performance obligation | Obligation								1
Period for expects performance obligation								5 years
Revenue recognized					2,100,000	$ 1,700,000		$ 7,300,000	$ 4,600,000
Deferred revenue					12,006,000			12,006,000		$ 15,840,000
Research Collaboration and License Agreement with Eli Lilly | Prepaid and Other Assets
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration receivables					$ 900,000			900,000		$ 1,200,000
Research Collaboration and License Agreement with Eli Lilly | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Rights to antibody oligonucleotide conjugates for messengers RNA targets. | Product				6
Development milestone payment eligible to receive per target				$ 60,000,000.0
Regulatory milestone payment eligible to receive per target				140,000,000.0
Commercialization milestone payment eligible to receive				$ 205,000,000.0
Research Collaboration with MyoKardia, Inc.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized								$ 200,000