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Fair Value Measurements - Option-pricing model for the valuation of the preferred stock warrant liability (Details) - Preferred Stock Warrant Liability
Jul. 29, 2017
$ / shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Expected term (in years) 2 years
Fair value of underlying shares (in dollars per share) $ 25.09
Volatility 43.80%
Risk free interest rate 1.30%
Dividend yield 0.00%