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Stock-Based Compensation - Allocated Stock-based Compensation (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]    
Allocated share based compensation expense $ 395 $ 3,169
Research and development    
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]    
Allocated share based compensation expense 77 729
General and administrative    
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]    
Allocated share based compensation expense $ 318 $ 2,440