Property and Equipment, Net - Components of Company's Property and Equipment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Depreciation	$ 2,800	$ 2,700	$ 5,600	$ 4,500
Property and equipment, gross	94,285		94,285		$ 94,248
Accumulated depreciation	(57,533)		(57,533)		(54,734)
Property and equipment, net	36,752		36,752		39,514
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	47,592		47,592		48,216
Office Furniture, Fixtures and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	18,159		18,159		17,732
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 28,534		$ 28,534		$ 28,300