Property and Equipment - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Accumulated depreciation and amortization	$ (54,776)	$ (41,614)
Property and equipment, net	55,535	43,988
Property and equipment, gross
Property, Plant and Equipment [Line Items]
Property and equipment, gross	102,451	79,909
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	33,467	29,558
Website and software development costs
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 46,014	33,278
Website and software development costs | Minimum
Property, Plant and Equipment [Line Items]
Useful Lives (Years)	2 years
Website and software development costs | Maximum
Property, Plant and Equipment [Line Items]
Useful Lives (Years)	3 years
Computer and office equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 13,154	7,765
Computer and office equipment | Minimum
Property, Plant and Equipment [Line Items]
Useful Lives (Years)	3 years
Computer and office equipment | Maximum
Property, Plant and Equipment [Line Items]
Useful Lives (Years)	5 years
Software
Property, Plant and Equipment [Line Items]
Useful Lives (Years)	3 years
Property and equipment, gross	$ 1,871	1,858
Furniture
Property, Plant and Equipment [Line Items]
Useful Lives (Years)	7 years
Property and equipment, gross	$ 7,945	7,450
Construction in progress
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 7,860	$ 5,693