Commitments and Contingencies - Additional Information (Detail)						1 Months Ended					3 Months Ended	12 Months Ended
	Oct. 24, 2018 USD ($)	May 01, 2018 USD ($) ft²	Oct. 06, 2017 ft²	Apr. 07, 2017 USD ($) ft²	Sep. 21, 2016 ft²	Dec. 31, 2017 USD ($)	Feb. 28, 2017 USD ($)	Nov. 30, 2016 Term	Jul. 31, 2015 ft²	May 31, 2013 USD ($)	Mar. 31, 2019 USD ($)	Oct. 31, 2017 USD ($)	Dec. 31, 2018	Sep. 30, 2015 ft²
Commitment And Contingencies [Line Items]
Operating lease term										42 months
Minimum monthly lease payments										$ 17,588
Operating lease, straight-line cost											$ 414,000
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											8.24%
Operating lease, remaining lease term											8 years 7 months 6 days
Operating lease, straight-line cost											$ 45,000
Manufacturing agreement period									10 years
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]
Minimum monthly lease payments		$ 87,000										$ 80,000
Additional leased office space | ft²			6,450	1,471					4,700
Extended leased office space | ft²									5,400					5,400
Office space leased on temporary agreement | ft²					6,700				6,700
Lease termination date		Oct. 31, 2023		Oct. 31, 2023					Oct. 31, 2019
Temporary lease termination date					Oct. 31, 2017
Operating lease office space | ft²		36,500		28,600
Lease payment per month for additional office space				$ 4,100
Operating lease commencement date			2018-04
Operating lease, discount rate											8.90%
Operating lease, remaining lease term											4 years 7 months 6 days		4 years 9 months 18 days
Operating lease, straight-line cost											$ 300,000
Woburn, Massachusetts [Member]
Commitment And Contingencies [Line Items]
Operating lease term							5 years
Operating lease, discount rate													8.40%
Operating lease, remaining lease term											2 years 10 months 24 days		3 years 2 months 12 days
Operating lease, straight-line cost											$ 46,000
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.