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