Net Loss Per Share - Reconciliation (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Net loss attributable to common stockholders	$ (30.9)	$ (31.1)	$ (88.4)	$ (75.8)
Denominator:
Weighted average common shares outstanding—basic and diluted (shares)	53,997,315	11,178,924	25,935,362	11,079,303
Net loss per share attributable to common stockholders—basic and diluted (usd per share)	$ (0.57)	$ (2.78)	$ (3.41)	$ (6.84)