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Fair Value Measurements - Common Stock Warrants (Details) - $ / shares
9 Months Ended 12 Months Ended
Sep. 30, 2019
Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Weighted average exercise price (in usd per share) $ 0.09 $ 0.09
Term (years) 2 years 11 months 1 day 3 years 8 months 1 day
Fair value of common stock warrants (in usd per share) $ 22.78 $ 14.13
Warrant    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Common stock warrants outstanding (in shares) 354,350 354,350