Restricted Cash - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash balance	$ 408	$ 769
Restricted cash in current assets	361
Stamford Lease [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Letter of credit, remaining amount	769
Letter of credit, through the end of lease term	$ 408
Letter of credit, remaining amount date	2019-05
Operating lease, expiration year	2023
Restricted cash balance	$ 408	$ 769
Restricted cash in current assets	$ 361