Property and Equipment - Components of Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 20,275	$ 19,617
Less: Accumulated depreciation	(17,387)	(15,404)
Property and equipment, net	2,888	4,213
Furniture and Equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	16,208	15,583
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	3,877	3,844
Aircraft
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 190	$ 190