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Preferred Stock and Common Stock Warrants - Assumptions (Details) - Common Stock Warrants - $ / shares
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Class of Warrant or Right [Line Items]    
Stock price $ 5.52 $ 5.40
Exercise price $ 0.01 $ 0.01
Risk-free rate 2.30% 2.50%
Volatility 29.20% 33.10%
Term (years) 2 years 6 months 2 years 8 months 12 days