Redeemable Convertible Preferred Stock - Summary of Convertible Preferred Stock (Details) - USD ($) $ / shares in Units, $ 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			35,962,362
Redeemable convertible preferred stock, shares issued			20,390,095
Redeemable convertible preferred stock, shares outstanding			20,390,095
Redeemable convertible preferred stock value			$ 160,310
Redeemable convertible preferred stock, liquidation value			$ 172,037
Series A Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized	10,502,804		10,502,804
Redeemable convertible preferred stock, shares issued	0		6,225,719
Redeemable convertible preferred stock, shares outstanding	0	6,225,719	6,225,719	6,225,719	6,225,719	6,225,719	6,225,719
Public offering price per share			$ 4.32
Redeemable convertible preferred stock value		$ 24,967	$ 24,967	$ 24,967	$ 24,967	$ 24,967	$ 24,967
Redeemable convertible preferred stock, liquidation value	$ 0		$ 26,887
Series B Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized	11,449,515		11,449,515
Redeemable convertible preferred stock, shares issued	0		6,786,896
Redeemable convertible preferred stock, shares outstanding	0	6,786,896	6,786,896	6,786,896	6,786,896	6,786,896	6,786,896
Public offering price per share			$ 8.86
Redeemable convertible preferred stock value		$ 55,151	$ 55,151	$ 55,151	$ 55,151	$ 55,151	$ 55,151
Redeemable convertible preferred stock, liquidation value	$ 0		$ 60,150
Series C Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares authorized	12,545,824		14,010,043
Redeemable convertible preferred stock, shares issued	0		7,377,480
Redeemable convertible preferred stock, shares outstanding	0	7,377,480	7,377,480	3,688,740	3,688,740	3,688,740	3,688,740
Public offering price per share			$ 11.52
Redeemable convertible preferred stock value		$ 80,192	$ 80,192	$ 37,692	$ 37,692	$ 37,692	$ 37,692
Redeemable convertible preferred stock, liquidation value	$ 0		$ 85,000