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Summary of Significant Accounting Policies - Stock-Based Compensation (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stock-Based Compensation      
Stock-based compensation expense $ 2,007 $ 2,160 $ 1,658
Tax benefits recognized from stock-based compensation expense 0 0 0
Research and development      
Stock-Based Compensation      
Stock-based compensation expense 784 723 681
General and administrative      
Stock-Based Compensation      
Stock-based compensation expense $ 1,223 $ 1,437 $ 977