Redeemable Convertible Preferred Stock - Summary of Redeemable Convertible Preferred Stock (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized	10,000,000		0
Redeemable convertible preferred stock, shares issued	0		0
Redeemable convertible preferred stock, shares outstanding	0		0
Series A Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized	0		56,000,000
Redeemable convertible preferred stock, shares issued	0		56,000,000
Redeemable convertible preferred stock, shares outstanding	0	56,000,000	56,000,000	56,000,000	56,000,000	56,000,000	56,000,000
Redeemable convertible preferred stock. aggregate liquidation preference	$ 0		$ 62,468
Redeemable convertible preferred stock value		$ 62,468	$ 62,468	$ 61,516	$ 61,516	$ 61,516	$ 61,516
Series B Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized	0		49,501,221
Redeemable convertible preferred stock, shares issued	0		49,501,221
Redeemable convertible preferred stock, shares outstanding	0	49,501,221	49,501,221	49,501,221	49,501,221	49,501,221	49,501,221
Redeemable convertible preferred stock. aggregate liquidation preference	$ 0		$ 75,860
Redeemable convertible preferred stock value		$ 75,860	$ 75,860	$ 74,665	$ 73,290	$ 71,931	$ 70,587
Series C Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized	0		44,000,000
Redeemable convertible preferred stock, shares issued	0		26,360,745
Redeemable convertible preferred stock, shares outstanding	0	54,888,058	26,360,745
Redeemable convertible preferred stock. aggregate liquidation preference	$ 0		$ 47,947
Redeemable convertible preferred stock value		$ 99,985	$ 47,947
Amended and Restated Certificate of Incorporation
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized			149,501,221
Redeemable convertible preferred stock, shares issued			131,861,966
Redeemable convertible preferred stock, shares outstanding			131,861,966
Redeemable convertible preferred stock. aggregate liquidation preference			$ 186,275
Redeemable convertible preferred stock value			$ 186,275
Amended and Restated Certificate of Incorporation | Series A Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized			56,000,000
Redeemable convertible preferred stock, shares issued			56,000,000
Redeemable convertible preferred stock, shares outstanding			56,000,000
Redeemable convertible preferred stock. aggregate liquidation preference			$ 62,468
Redeemable convertible preferred stock value			$ 62,468
Amended and Restated Certificate of Incorporation | Series B Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized			49,501,221
Redeemable convertible preferred stock, shares issued			49,501,221
Redeemable convertible preferred stock, shares outstanding			49,501,221
Redeemable convertible preferred stock. aggregate liquidation preference			$ 75,860
Redeemable convertible preferred stock value			$ 75,860
Amended and Restated Certificate of Incorporation | Series C Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized			44,000,000
Redeemable convertible preferred stock, shares issued			26,360,745
Redeemable convertible preferred stock, shares outstanding			26,360,745
Redeemable convertible preferred stock. aggregate liquidation preference			$ 47,947
Redeemable convertible preferred stock value			$ 47,947