Overview and Basis of Presentation (Details) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended
		Sep. 30, 2018 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) segment shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($)
Class of Stock [Line Items]
Aggregate net proceeds | $				$ 245,985
Offering costs | $				5,450
Conversion of redeemable convertible preferred stock in connection with initial public offering (in shares)		41,628,207
Increase in marketing and support expense | $	[1]		$ 102,874	83,428	$ 59,740
Decrease in general and administrative expense | $	[1]		$ (100,541)	$ (80,134)	(62,989)
Number of reportable segments | segment			1
Number of operating segments | segment			1
Reclassification Adjustment
Class of Stock [Line Items]
Increase in marketing and support expense | $			$ 13,600		4,600
Decrease in general and administrative expense | $			$ 13,600		$ 4,600
Series G Redeemable Convertible Preferred Stock
Class of Stock [Line Items]
Outstanding warrants to purchase Series G redeemable preferred stock (in shares)		933,269
Conversion of Redeemable Convertible Preferred Stock
Class of Stock [Line Items]
Conversion of redeemable convertible preferred stock in connection with initial public offering (in shares)				41,628,207
IPO
Class of Stock [Line Items]
Offering price (in dollars per share) | $ / shares		$ 23.00
Aggregate net proceeds | $		$ 246,000
Offering costs | $		$ 5,500
Class A Common Stock | Common Stock
Class of Stock [Line Items]
Conversion of convertible stock (in shares)			43,255,565
Class A Common Stock | IPO
Class of Stock [Line Items]
Shares issued in initial public offering (in shares)		11,500,000
Offering price (in dollars per share) | $ / shares		$ 23.00
Class A Common Stock | Over-Allotment Option
Class of Stock [Line Items]
Shares issued in initial public offering (in shares)		1,500,000
Class B Common Stock | Common Stock
Class of Stock [Line Items]
Conversion of convertible stock (in shares)			(43,255,565)	42,188,624
Class B Common Stock | Common Stock | Conversion of Redeemable Convertible Preferred Stock
Class of Stock [Line Items]
Conversion of convertible stock (in shares)		42,188,624
Class B Common Stock | Common Stock | Conversion of Warrants
Class of Stock [Line Items]
Conversion of convertible stock (in shares)		997,193

[1] (1) Includes stock-based compensation as follows (in thousands):Year Ended December 31,201920182017Cost of net revenue$1,397  $429  $200  Product development11,130  5,813  2,411  Sales, marketing and support5,471  3,570  2,364  General and administrative19,596  20,419  5,883