Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Depreciation [Abstract]
Depreciation expense	$ 1.9	$ 1.5	$ 1.5
Book value of property and equipment under debt agreement	$ 7.4	$ 5.5