Annual report pursuant to Section 13 and 15(d)

Stock-Based Payments - Weighted-Average Assumptions to Estimate Fair Value of Stock Options (Details)

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Stock-Based Payments - Weighted-Average Assumptions to Estimate Fair Value of Stock Options (Details) - Stock Option - $ / shares
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Weighted-average risk-free interest rate 2.42% 2.56% 2.07%
Expected option term (in years) 6 years 6 years 14 days 6 years 18 days
Volatility 91.35% 90.26% 87.83%
Weighted-average grant date fair value of options granted (in dollars per share) $ 5.05 $ 7.80 $ 8.75