Balance Sheet Components - Property and Equipment, Net (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 38,642		$ 38,642		$ 29,369
Less: accumulated depreciation	(11,358)		(11,358)		(7,915)
Property and equipment, net	27,284		27,284		21,454
Depreciation expense	800	$ 600	1,500	$ 1,400
Capitalized internal-use software and platform development costs	1,700	$ 1,400	3,300	$ 2,500
Amortization of internal-use software and platform development costs	1,100		2,000
Computer equipment and software
Property, Plant and Equipment [Line Items]
Total property and equipment	4,664		4,664		3,613
Internal-use software and platform development
Property, Plant and Equipment [Line Items]
Total property and equipment	16,027		16,027		12,726
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	14,597		14,597		10,576
Office furniture and fixtures
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 3,354		$ 3,354		$ 2,454