Commitments and Contingencies - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2019	Dec. 31, 2018	May 31, 2016
Other Commitments [Line Items]
Deferred lease obligation				$ 1,562,000
Operating lease liability	$ 4,988,000
Stamford Operating Lease [Member]
Other Commitments [Line Items]
Operating Lease, renewable term	5 years
Operating lease, expiration date	Nov. 30, 2023
Operating Lease, description	The Stamford Lease requires monthly lease payments, including rent escalations and rent holidays, during the initial lease term. The Company began to make rental payments from the Commencement Date. Prior to January 1, 2019, the Company recorded monthly rent expense on a straight-line basis from March 2016, upon taking possession of the Premises, through December 31, 2018. As of December 31, 2018, the balance of deferred lease obligation, representing the difference between cash rent paid and straight-line rent expense, was $864.
Deferred lease obligation				864,000
Tenant improvement expenses					$ 1,094,000
Lease, rent expense		$ 246,000
Tenant improvement incentives				698,000
Operating lease cost	$ 234,000
Stamford Operating Lease [Member] | Research and Development [Member]
Other Commitments [Line Items]
Operating lease cost	164,000
Stamford Operating Lease [Member] | General and Administrative [Member]
Other Commitments [Line Items]
Operating lease cost	$ 70,000
Stamford Operating Lease [Member] | ASC 842 [Member]
Other Commitments [Line Items]
Operating Lease, renewable term	5 years
Operating lease liability			$ 5,198,000
Stamford Operating Lease [Member] | Standby Letter of Credit [Member]
Other Commitments [Line Items]
Letter of credit, amount	$ 769,000
Operating lease, renewable term description	Automatically renewed annually through November 2023.
Stamford Operating Lease [Member] | Standby Letter of Credit [Member] | Maximum [Member]
Other Commitments [Line Items]
Letter of credit, renewal date	Nov. 30, 2023
Stamford Operating Lease [Member] | Leasehold Improvements [Member]
Other Commitments [Line Items]
Deferred lease obligation				$ 698,000