Annual report pursuant to Section 13 and 15(d)

Stock-Based Payments - Stock-Based Compensation Expense (Details)

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Stock-Based Payments - Stock-Based Compensation Expense (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]      
Stock-based compensation expense $ 6,610 $ 4,419 $ 4,234
Stock options      
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]      
Unrecognized compensation costs $ 14,800    
Period in which compensation costs will be recognized 3 years    
Performance-based stock options      
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]      
Unrecognized compensation costs $ 1,000    
Period in which compensation costs will be recognized 3 years 9 months 18 days    
Research and development      
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]      
Stock-based compensation expense $ 2,412 1,666 2,980
General and administrative      
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]      
Stock-based compensation expense $ 4,198 $ 2,753 $ 1,254