Basis of Presentation and Summary of Significant Accounting Policies - Narrative (Details) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Class of Stock [Line Items]
Offering costs				$ 3,734,000	$ 0
Deferred offering costs	$ 5,500,000	$ 5,500,000		5,500,000		$ 0
Offering costs reclassified as a reduction of proceeds received from IPO	5,500,000
Stock-based compensation expense	$ 21,000,000.0	$ 22,911,000	$ 24,436,000	$ 25,012,000	26,285,000
Redeemable convertible preferred stock, shares issued (in shares)	0	0		0		165,658,000
Redeemable convertible preferred stock, shares outstanding (in shares)	0	0		0		165,658,000
Options
Class of Stock [Line Items]
Vesting period				4 years
Restricted Stock Units (RSUs)
Class of Stock [Line Items]
Vesting period				4 years
Stock-based compensation expense		$ 15,400,000	0	$ 15,400,000	0
Class A common stock
Class of Stock [Line Items]
Conversion of redeemable convertible preferred stock to common stock upon initial public offering (in shares)	31,381,152
Class B common stock
Class of Stock [Line Items]
Conversion of redeemable convertible preferred stock to common stock upon initial public offering (in shares)	134,276,690
Stock-based compensation expense			$ 23,300,000		$ 23,300,000
Series D, Redeemable Convertible Preferred Stock
Class of Stock [Line Items]
Conversion of redeemable convertible preferred stock to common stock upon initial public offering (in shares)	(31,381,152)
Series A, Series B, And Series C, Redeemable Convertible Preferred Stock
Class of Stock [Line Items]
Conversion of redeemable convertible preferred stock to common stock upon initial public offering (in shares)	(134,276,690)
Initial Public Offering
Class of Stock [Line Items]
Stock-based compensation expense	$ 21,000,000.0
Initial Public Offering | Class A common stock
Class of Stock [Line Items]
Shares issued in initial public offering (in shares)	40,250,000
Aggregate proceeds received from initial public offering, net of underwriters' discounts and commissions	$ 570,500,000
Offering costs	$ 5,500,000
Underwriters' Option | Class A common stock
Class of Stock [Line Items]
Shares issued in initial public offering (in shares)	5,250,000
Price per share in initial public offering (in dollars per share)	$ 15.00	$ 15.00		$ 15.00
Aggregate proceeds received from initial public offering, net of underwriters' discounts and commissions	$ 74,400,000
Former Employees | Class A common stock
Class of Stock [Line Items]
Conversion of redeemable convertible preferred stock to common stock upon initial public offering (in shares)	15,198,587
Former Employees | Class B common stock
Class of Stock [Line Items]
Conversion of redeemable convertible preferred stock to common stock upon initial public offering (in shares)	(15,198,587)