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Fair Value Measurements - Summary of Fair Value of Series B Preferred Stock Tranche Rights Liability (Details) - Series B Preferred Stock - Significant unobservable inputs (Level 3)
€ in Millions, $ in Millions
Mar. 23, 2020
USD ($)
Dec. 31, 2019
USD ($)
Sep. 30, 2019
USD ($)
Sep. 30, 2019
EUR (€)
Dec. 31, 2018
USD ($)
Dec. 31, 2018
EUR (€)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Implied equity value   $ 74.4 $ 55.6 € 48.8 $ 35.4 € 30.8
Tranche Three Settlement            
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Implied equity value $ 208.2          
Tranche 2 milestone (by March 31, 2019)            
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Probability of success of reaching necessary milestone         85.00% 85.00%
Tranche 3 milestone (by March 31, 2020)            
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Probability of success of reaching necessary milestone   90.00% 90.00% 90.00% 90.00% 90.00%
Tranche 3 milestone (by March 31, 2020) | Tranche Three Settlement            
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Probability of success of reaching necessary milestone 90.00%          
Expected industry return over period during which milestones are expected to be achieved            
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Measurement input   15.0 15.0 15.0 15.0 15.0
Expected industry return over period during which milestones are expected to be achieved | Tranche Three Settlement            
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Measurement input 13.0          
Risk-free interest rate            
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Measurement input   2.3 1.9 1.9 2.5 2.5
Risk-free interest rate | Tranche Three Settlement            
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]            
Measurement input 1.1