Property and Equipment - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property Plant And Equipment [Abstract]
Disposal of long-lived assets	$ 0	$ 1,000,000	$ 0
Depreciation	$ 2,486,000	$ 1,942,000	$ 1,248,000