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Stock-Based Compensation - Summary of Stock-Based Compensation Expense (Details) - Employees And Non Employees - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Total stock-based compensation expense $ 663 $ 1,181
Research and Development    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Total stock-based compensation expense 277 625
General and Administrative    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Total stock-based compensation expense $ 386 $ 556