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Summary of Significant Accounting Policies - Stock-Based Compensation (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Stock-Based Compensation    
Stock-based compensation expense $ 479 $ 505
Research and development    
Stock-Based Compensation    
Stock-based compensation expense 173 200
General and administrative    
Stock-Based Compensation    
Stock-based compensation expense $ 306 $ 305