Commitments and Contingencies - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	May 31, 2016
Other Commitments [Line Items]
Deferred lease obligation	$ 1,657,000	$ 1,718,000
Stamford Operating Lease [Member]
Other Commitments [Line Items]
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. The Company records monthly rent expense on a straight-line basis from March 2016, upon taking possession of the Premises, through November 2023. As of March 31, 2018 and December 31, 2017, the balance of deferred lease obligation, representing the difference between cash rent paid and straight-line rent expense, was $852 and $876, respectively. The Stamford Lease is renewable for one five-year term.
Operating Lease, renewable term	5 years
Operating lease, expiration date	Nov. 30, 2023
Deferred lease obligation	$ 852,000	876,000
Tenant improvement expenses			$ 1,094,000
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	$ 805,000	$ 842,000