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Stock Incentive Plans - Stock-Based Compensation Expense (Details)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Assumptions used to calculate fair value of stock options    
Expected dividend yield 0.00% 0.00%
Minimum    
Assumptions used to calculate fair value of stock options    
Weighted-average expected volatility 68.90% 73.70%
Expected term (in years) 5 years 6 months 5 years 9 months
Risk-free interest rate 1.39% 2.67%
Maximum    
Assumptions used to calculate fair value of stock options    
Weighted-average expected volatility 110.50% 76.90%
Expected term (in years) 10 years 6 years 3 months
Risk-free interest rate 2.69% 2.84%