Property and Equipment - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 36,456	$ 32,500
Accumulated depreciation and amortization	(14,138)	(13,274)
Property and equipment, net	$ 22,318	19,226
Minimum
Property, Plant and Equipment [Line Items]
Useful Lives	2 years
Maximum
Property, Plant and Equipment [Line Items]
Useful Lives	7 years
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 16,039	4,911
Website and software development costs
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 14,501	10,114
Website and software development costs | Minimum
Property, Plant and Equipment [Line Items]
Useful Lives	1 year
Website and software development costs | Maximum
Property, Plant and Equipment [Line Items]
Useful Lives	3 years
Computer and office equipment
Property, Plant and Equipment [Line Items]
Useful Lives	3 years
Property and equipment, gross	$ 2,192	2,846
Software
Property, Plant and Equipment [Line Items]
Useful Lives	3 years
Property and equipment, gross	$ 685	1,367
Furniture
Property, Plant and Equipment [Line Items]
Useful Lives	7 years
Property and equipment, gross	$ 3,039	2,406
Construction in progress
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 0	$ 10,856