Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 82,902	$ 65,520
Less accumulated depreciation and amortization	(30,989)	(22,226)
Property and equipment, net	51,913	43,294
Computer Equipment and Purchased Software [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	7,336	4,571
Employee Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	7,685	4,260
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	12,703	11,083
Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	2,537	2,620
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	43,826	33,446
Equipment under Capital Lease [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	3,450	3,450
Internal-Use Software [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	4,784	2,892
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 581	$ 3,198