Redeemable Convertible Preferred Stock - Summary of Redeemable Convertible Preferred Stock Outstanding (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Redeemable convertible preferred stock, Shares Authorized			75,238,150
Redeemable convertible preferred stock, Shares Issued			18,474,742
Redeemable convertible preferred stock, Shares Outstanding			18,474,742
Redeemable convertible preferred stock, Aggregate Liquidation Preference			$ 89,784
Redeemable convertible preferred stock, Net Carrying Value	$ 0	$ 0	$ 89,404	$ 89,404	$ 75,995	$ 75,995
Series A Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable convertible preferred stock, Shares Authorized			31,250,000
Redeemable convertible preferred stock, Shares Issued			7,812,497
Redeemable convertible preferred stock, Shares Outstanding			7,812,497
Redeemable convertible preferred stock, Aggregate Liquidation Preference			$ 20,500
Redeemable convertible preferred stock, Net Carrying Value			$ 20,261
Series B Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable convertible preferred stock, Shares Authorized			19,288,150
Redeemable convertible preferred stock, Shares Issued			4,799,548
Redeemable convertible preferred stock, Shares Outstanding			4,799,548
Redeemable convertible preferred stock, Aggregate Liquidation Preference			$ 22,078
Redeemable convertible preferred stock, Net Carrying Value			$ 22,047
Series C Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable convertible preferred stock, Shares Authorized			24,700,000
Redeemable convertible preferred stock, Shares Issued			5,862,697
Redeemable convertible preferred stock, Shares Outstanding			5,862,697
Redeemable convertible preferred stock, Aggregate Liquidation Preference			$ 47,206
Redeemable convertible preferred stock, Net Carrying Value			$ 47,096