Convertible Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Temporary Equity [Line Items]
Conversion of Convertible Preferred Stock to common stock upon closing of initial public offering (shares)	31,557,107	31,557,107
Convertible preferred stock, authorized (shares)	0	0			31,557,107
Convertible preferred stock, par value (usd per share)	$ 0.00001	$ 0.00001			$ 0.00001
Convertible preferred stock, outstanding (shares)	0	0	31,557,107	31,557,107	31,557,107	31,557,107	28,595,471,000,000	24,449,177	24,445,555
Convertible preferred stock, issued (shares)	0	0			31,557,107
Convertible preferred stock, carrying value	$ 0.0	$ 0.0	$ 480.2	$ 480.2	$ 480.2	$ 480.2	$ 355.2	$ 180.8	$ 180.8
Convertible preferred stock, liquidation preference	$ 0.0	$ 0.0			$ 480.8
Convertible preferred stock, common stock issuable upon conversion (shares)					31,557,107
Series Seed Preferred stock
Temporary Equity [Line Items]
Convertible preferred stock, authorized (shares)					7,905,140
Convertible preferred stock, outstanding (shares)					7,905,140
Convertible preferred stock, issued (shares)					7,905,140
Convertible preferred stock, carrying value					$ 12.9
Convertible preferred stock, liquidation preference					$ 13.0
Convertible preferred stock, common stock issuable upon conversion (shares)					7,905,140
Series A Preferred stock
Temporary Equity [Line Items]
Convertible preferred stock, authorized (shares)					3,328,774
Convertible preferred stock, outstanding (shares)					3,328,774
Convertible preferred stock, issued (shares)					3,328,774
Convertible preferred stock, carrying value					$ 14.0
Convertible preferred stock, liquidation preference					$ 13.6
Convertible preferred stock, common stock issuable upon conversion (shares)					3,328,774
Series B Preferred stock
Temporary Equity [Line Items]
Convertible preferred stock, authorized (shares)					4,511,417
Convertible preferred stock, outstanding (shares)					4,511,417
Convertible preferred stock, issued (shares)					4,511,417
Convertible preferred stock, carrying value					$ 34.1
Convertible preferred stock, liquidation preference					$ 34.1
Convertible preferred stock, common stock issuable upon conversion (shares)					4,511,417
Series C Preferred stock
Temporary Equity [Line Items]
Convertible preferred stock, authorized (shares)					8,703,846
Convertible preferred stock, outstanding (shares)					8,703,846
Convertible preferred stock, issued (shares)					8,703,846
Convertible preferred stock, carrying value					$ 119.8
Convertible preferred stock, liquidation preference					$ 120.1
Convertible preferred stock, common stock issuable upon conversion (shares)					8,703,846
Series D Preferred stock
Temporary Equity [Line Items]
Convertible preferred stock, authorized (shares)					7,107,930
Convertible preferred stock, outstanding (shares)					7,107,930
Convertible preferred stock, issued (shares)					7,107,930
Convertible preferred stock, carrying value					$ 299.4
Convertible preferred stock, liquidation preference					$ 300.0
Convertible preferred stock, common stock issuable upon conversion (shares)					7,107,930