Fixed Assets, net - Schedule of Fixed Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Less accumulated depreciation and amortization	$ (5,558)	$ (4,672)
Total	9,723	10,693
Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Gross	2,028	1,879
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Gross	2,386	2,449
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Gross	$ 10,867	$ 11,037