Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Depreciation and amortization expense	$ 495	$ 1,465	$ 839
Net proceeds from sale of property and equipment	41
Shelton Operating Lease [Member]
Property, Plant and Equipment [Line Items]
Wrote-off of fully-depreciated Shelton property and equipment	7,816	$ 397
Net proceeds from sale of property and equipment	$ 41