Balance Sheet Components - Property and Equipment, Net (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 41,028		$ 41,028		$ 29,369
Less: accumulated depreciation	(13,348)		(13,348)		(7,915)
Property and equipment, net	27,680		27,680		21,454
Depreciation expense	1,000	$ 700	2,500	$ 2,100
Capitalized internal-use software and platform development costs	2,300	2,000	5,600	4,500
Amortization of internal-use software and platform development costs	1,000	$ 300	3,000	$ 400
Computer equipment and software
Property, Plant and Equipment [Line Items]
Total property and equipment	4,780		4,780		3,613
Internal-use software and platform development
Property, Plant and Equipment [Line Items]
Total property and equipment	18,282		18,282		12,726
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	14,612		14,612		10,576
Office furniture and fixtures
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 3,354		$ 3,354		$ 2,454