Property and Equipment, Net - Components of Company's Property and Equipment (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Depreciation	$ 2,800	$ 1,800
Property and equipment, gross	94,690		$ 94,248
Accumulated depreciation	(56,226)		(54,734)
Property and equipment, net	38,464		39,514
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	47,472		48,216
Office Furniture, Fixtures and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	18,240		17,732
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 28,978		$ 28,300