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Significant Agreements - Additional Information (Detail)			1 Months Ended				3 Months Ended								12 Months Ended
	Dec. 18, 2018	Sep. 27, 2018 USD ($) Milestone	Sep. 30, 2018 USD ($)	May 31, 2017 USD ($)	Apr. 30, 2017 USD ($) shares	Dec. 31, 2015 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Increase (decrease) in deferred revenue															$ (520,000)	$ (444,000)
Recognition of revenue							$ 111,000	$ 1,801,000	$ 254,000	$ 354,000	$ 111,000	$ 111,000	$ 2,111,000	$ 111,000	2,520,000	2,444,000
Deferred revenue included in current liabilities							268,000				$ 444,000				268,000	444,000
AbbVie Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of mile stone payments under second phase | Milestone		2
Initial milestone payment upon execution of amendment		$ 2,000,000
Increase in transaction price of performance obligation of second phase		$ 2,000,000
Cumulative catch-up adjustment to revenue, contract modification			$ 1,800,000
Increase (decrease) in deferred revenue			$ 300,000
Recognition of revenue															2,500,000	$ 2,400,000
Deferred revenue included in current liabilities							$ 300,000								300,000
AbbVie Agreement | IBDCo
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront non refundable payment						$ 2,000,000
Potential milestone payment															$ 16,500,000
Collaboration in research and development															54 months
Milestone payment				$ 2,000,000
Massachusetts Institute of Technology and Boston University License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Type of Cost, Good or Service [Extensible List]					us-gaap:LicenseAndServiceMember
Recognition of license fees					$ 1,800,000
Payment for prior patent costs					300,000
Massachusetts Institute of Technology and Boston University License Agreement | Intellectual Property
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License acquired					$ 50,000
Massachusetts Institute of Technology and Boston University License Agreement | Common Stock | 2017 Reorganization
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock, converted | shares					325,377
Massachusetts Institute of Technology and Boston University License Agreement | Common units
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common units issued for license | shares					325,377
Massachusetts Institute of Technology and Boston University License Agreement | Mirna therapeutics | Common Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock, converted | shares					179,999
Boston University License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License agreement termination effective date	Feb. 16, 2019
Massachusetts Institute of Technology License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License agreement termination effective date	Jun. 19, 2019

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