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Summary of Significant Accounting Policies - Stock-Based Compensation (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018
Stock Incentive Plans |  |
Stock-based compensation expense | $ 1,512,000 | $ 1,771,000
Tax benefits recognized from stock-based compensation expense | 0 | 0
Research and development |  |
Stock Incentive Plans |  |
Stock-based compensation expense | 616,000 | 673,000
General and administrative |  |
Stock Incentive Plans |  |
Stock-based compensation expense | $ 896,000 | $ 1,098,000

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