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Share-Based Compensation - Share-based Compensation Expense (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation [Line Items]    
Share-based compensation expense $ 1,295 $ 2,453
Research and development    
Share-based Compensation [Line Items]    
Share-based compensation expense 274 446
General and administrative    
Share-based Compensation [Line Items]    
Share-based compensation expense $ 1,021 $ 2,007