Property and Equipment - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Abstract]
Depreciation expense related to plant and equipment	$ 361	$ 590	$ 618
Amount disposed of property plant and equipment	325	1,000	238
Gain or loss on disposal of property plant and equipment	$ (30)	$ 13	$ 10