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Preferred and Common Stock Warrants - Assumptions Used to Calculate Fair Value of Outstanding Warrants (Details) - Redeemable Convertible Preferred Stock Warrants
Dec. 31, 2017
Dec. 31, 2016
Dividend Yield    
Class of Warrant or Right [Line Items]    
Outstanding warrants measurement input 0 0
Expected Term    
Class of Warrant or Right [Line Items]    
Outstanding warrants measurement input 2 years 9 months  
Expected Term | Minimum    
Class of Warrant or Right [Line Items]    
Outstanding warrants measurement input   6 months
Expected Term | Maximum    
Class of Warrant or Right [Line Items]    
Outstanding warrants measurement input   3 years 9 months
Risk-Free Interest Rates    
Class of Warrant or Right [Line Items]    
Outstanding warrants measurement input 0.018  
Risk-Free Interest Rates | Minimum    
Class of Warrant or Right [Line Items]    
Outstanding warrants measurement input   0.010
Risk-Free Interest Rates | Maximum    
Class of Warrant or Right [Line Items]    
Outstanding warrants measurement input   0.016
Expected Volatility    
Class of Warrant or Right [Line Items]    
Outstanding warrants measurement input 0.346  
Expected Volatility | Minimum    
Class of Warrant or Right [Line Items]    
Outstanding warrants measurement input   0.348
Expected Volatility | Maximum    
Class of Warrant or Right [Line Items]    
Outstanding warrants measurement input   0.392