Restricted Cash - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash in current assets		$ 700
Restricted cash balance	$ 769	769
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 date	Nov. 30, 2023
Restricted cash balance	$ 769	769
Shelton Lease [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash in current assets		$ 700