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Stock-based Compensation - Fair Value Disclosures (Details)
$ in Millions
12 Months Ended
Dec. 31, 2018
USD ($)
Stock-based compensation  
Risk-free interest rate (as a percent) 2.85%
Expected term 6 years 3 months
Expected dividends (as a percent) 0.00%
Expected volatility (as a percent) 121.52%
Stock options  
Stock-based compensation  
Fair value of stock options $ 3.5