Commitments and Contingencies - Additional Information (Detail)		1 Months Ended						3 Months Ended	6 Months Ended
	Oct. 24, 2018 USD ($)	Jun. 30, 2019 USD ($) ft²	Dec. 31, 2017 USD ($)	Feb. 28, 2017 USD ($)	Nov. 30, 2016 Term	Jul. 31, 2015	May 31, 2013 ft²	Jun. 30, 2019 USD ($) ft²	Jun. 30, 2019 USD ($) ft²	Jan. 01, 2019 USD ($)	Dec. 31, 2018
Commitment And Contingencies [Line Items]
Right-of-use assets		$ 6,493,000						$ 6,493,000	$ 6,493,000	$ 6,600,000
Operating lease liabilities										$ 7,000,000.0
Operating lease, straight-line cost								$ 417,000	831,000
GeneQuine Biotherapeutics GmbH ("GeneQuine") [Member] | Definitive Agreement [Member]
Commitment And Contingencies [Line Items]
License agreement, upfront fee paid			$ 2,000,000
Milestone, Revenue recognized									$ 0
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
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%						6.10%	6.10%
Operating lease, remaining lease term		8 years 3 months 18 days						8 years 3 months 18 days	8 years 3 months 18 days
Operating lease, straight-line cost								$ 48,000	$ 93,000
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
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											8.90%
Operating lease, remaining lease term		5 years 9 months 18 days						5 years 9 months 18 days	5 years 9 months 18 days		4 years 9 months 18 days
Additional leased office space | ft²		5,330						5,330	5,330
Minimum monthly lease payments									$ 108,000
Lease commencement date for additional space									Jul. 01, 2019
Right-of-use assets		$ 0						$ 0	$ 0
Operating lease liabilities		$ 0						0	0
Operating lease, straight-line cost								$ 300,000	$ 600,000
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 8 months 12 days						2 years 8 months 12 days	2 years 8 months 12 days		3 years 2 months 12 days
Operating lease, straight-line cost								$ 46,000	$ 92,000
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.