Redeemable Convertible Preferred Stock - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			3 Months Ended		9 Months Ended
	Sep. 30, 2019	Jan. 31, 2012	Oct. 31, 2011	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017	Apr. 30, 2011
Class of Stock [Line Items]
Conversion of redeemable convertible preferred stock to common stock upon initial public offering	$ 331,500			$ 331,521		$ 331,521
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
Preferred stock, shares authorized (in shares)	225,000,000			225,000,000		225,000,000
Preferred stock, par value (in dollars per share)	$ 0.001			$ 0.001		$ 0.001
Change in fair value of redeemable convertible preferred stock warrant liability				$ 1,200	$ 1,000	$ 1,517	$ 1,187
Warrant To Purchase Series B Redeemable Preferred Stock
Class of Stock [Line Items]
Number of shares subject to purchase from warrant (in shares)		23,760	94,510									59,140
Proceeds from drawdown		$ 400	$ 1,600
Exercise price of warrants (in dollars per share)												$ 0.34
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
Number of shares issued from exercise of warrants (in shares)				174,347		174,347
Redeemable convertible preferred stock
Class of Stock [Line Items]
Conversion of redeemable convertible preferred stock to common stock upon initial public offering (in shares)				(165,658,000)		(165,658,000)
Conversion of redeemable convertible preferred stock to common stock upon initial public offering	$ (331,500)			$ (331,521)		$ (331,521)
Redeemable convertible preferred stock, shares issued (in shares)	0			0		0
Redeemable convertible preferred stock, shares outstanding (in shares)	0			0	165,658,000	0	165,658,000	165,658,000	165,658,000	152,022,000	152,022,000
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)
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)
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