Collaboration, License and Research Agreements - Additional Information (Details)	1 Months Ended		3 Months Ended								12 Months Ended
	Nov. 30, 2019 shares	Apr. 30, 2019 USD ($) Product	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) Obligation	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized			$ 2,142,000	$ 1,746,000	$ 1,541,000	$ 1,358,000	$ 1,445,000	$ 650,000	$ 224,000	$ 0	$ 6,787,000	$ 2,319,000	$ 379,000
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
License agreement expiry period		10 years
Number of performance obligation | Obligation											1
Period for expects performance obligation											5 years
Revenue recognized											$ 6,800,000	2,300,000	0
Deferred revenue			15,840,000				18,940,000				15,840,000	18,940,000
Research Collaboration and License Agreement with Eli Lilly | Prepaid and Other Assets
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration receivables			$ 1,200,000				$ 800,000				1,200,000	$ 800,000
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
Regulatory milestone payment eligible to receive per target		140,000,000
Commercialization milestone payment eligible to receive		$ 205,000,000
Research Collaboration with MyoKardia, Inc.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized											$ 0
Agreements with Pharmaceutical Companies
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized													$ 400,000