Commitments and Contingencies - Additional Information (Detail)		1 Months Ended							3 Months Ended	9 Months Ended
	Oct. 24, 2018 USD ($)	Sep. 30, 2019 USD ($) ft²	Jun. 30, 2019 USD ($) ft²	Dec. 31, 2017 USD ($)	Feb. 28, 2017 USD ($)	Nov. 30, 2016 Term	Jul. 31, 2015	May 31, 2013 ft²	Sep. 30, 2019 USD ($) ft²	Sep. 30, 2019 USD ($) ft²	Jan. 01, 2019 USD ($)	Dec. 31, 2018
Commitment And Contingencies [Line Items]
Right-of-use assets		$ 8,618,000							$ 8,618,000	$ 8,618,000	$ 6,600,000
Operating lease liabilities											$ 7,000,000.0
Operating lease, straight-line cost									$ 567,000	1,398,000
GeneQuine Biotherapeutics GmbH ("GeneQuine") [Member] | Definitive Agreement [Member]
Commitment And Contingencies [Line Items]
License agreement, upfront fee paid				$ 2,000,000.0
Milestone, Revenue recognized										0
Research and development milestone payment	$ 750,000
GeneQuine Biotherapeutics GmbH ("GeneQuine") [Member] | Definitive Agreement [Member] | Investigational New Drug (IND) [Member]
Commitment And Contingencies [Line Items]
Research and development milestone payment				750,000
GeneQuine Biotherapeutics GmbH ("GeneQuine") [Member] | Definitive Agreement [Member] | Maximum [Member]
Commitment And Contingencies [Line Items]
Additional milestone payments				54,000,000.0
Xenon Pharmaceuticals, Inc. ("Xenon") [Member] | Definitive Agreement [Member]
Commitment And Contingencies [Line Items]
License agreement, upfront fee paid		3,000,000.0
Milestone, Revenue recognized										$ 0
Xenon Pharmaceuticals, Inc. ("Xenon") [Member] | Definitive Agreement [Member] | Sales Related Milestone Payments [Member]
Commitment And Contingencies [Line Items]
Additional milestone payments		75,000,000.0
Xenon Pharmaceuticals, Inc. ("Xenon") [Member] | Definitive Agreement [Member] | Maximum [Member]
Commitment And Contingencies [Line Items]
Additional milestone payments		$ 40,800,000
Evonik Corporation [Member]
Commitment And Contingencies [Line Items]
Supply agreement period						5 years
Supply agreement renewal term						2 years
Number of renewal terms | Term						2
Supply agreement description										The Supply Agreement will renew for two successive two year terms upon mutual written consent by both parties.
Manufacturing and Supply Agreement with Patheon UK Limited [Member]
Commitment And Contingencies [Line Items]
Operating lease, discount rate			6.10%
Operating lease, remaining lease term		8 years 1 month 6 days							8 years 1 month 6 days	8 years 1 month 6 days
Operating lease, straight-line cost									$ 54,000	$ 147,000
Manufacturing agreement period							10 years
Increase in operating lease liability			$ 500,000
Increase in right of use asset			$ 500,000
Phase 2 Proof of Concept (PoC) Clinical Trial [Member] | GeneQuine Biotherapeutics GmbH ("GeneQuine") [Member] | Definitive Agreement [Member] | Maximum [Member]
Commitment And Contingencies [Line Items]
Additional milestone payments				$ 8,700,000
Phase 2 Proof of Concept (PoC) Clinical Trial [Member] | Xenon Pharmaceuticals, Inc. ("Xenon") [Member] | Definitive Agreement [Member] | Maximum [Member]
Commitment And Contingencies [Line Items]
Additional milestone payments		$ 9,000,000.0
Burlington Massachusetts [Member]
Commitment And Contingencies [Line Items]
Operating lease office space | ft²		41,873						36,500	41,873	41,873
Lease termination date								Oct. 31, 2023		Apr. 30, 2025
Operating lease, discount rate		6.80%							6.80%	6.80%		8.90%
Operating lease, remaining lease term		5 years 7 months 6 days							5 years 7 months 6 days	5 years 7 months 6 days		4 years 9 months 18 days
Additional leased office space | ft²			5,330
Minimum monthly lease payments										$ 108,000
Lease commencement date for additional space										Jul. 01, 2019
Right-of-use assets		$ 2,500							$ 2,500	$ 2,500
Operating lease liabilities		$ 2,500							2,500	2,500
Operating lease, straight-line cost									$ 500,000	$ 1,100,000
Lessee, operating lease, extension term		18 months							18 months	18 months
Woburn, Massachusetts [Member]
Commitment And Contingencies [Line Items]
Operating lease, discount rate												8.40%
Operating lease, remaining lease term		2 years 4 months 24 days							2 years 4 months 24 days	2 years 4 months 24 days		3 years 2 months 12 days
Operating lease, straight-line cost									$ 46,000	$ 139,000
Operating lease term					5 years
Monthly lease payment					$ 15,000
Operating lease, option to extend										The Woburn lease includes an option to extend the term of the lease for two years.