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Merger - Additional Information (Details) - USD ($)
$ / shares in Units, $ in Thousands
Jun. 15, 2020
Sep. 30, 2020
Dec. 31, 2019
Business Acquisition [Line Items]      
Common stock, shares outstanding   11,216,563 2,108,266
Vested restricted stock awards outstanding   1,038,496  
Tocagen, Inc.      
Business Acquisition [Line Items]      
Estimated fair value of total consideration $ 32,981    
Common stock, shares outstanding 1,594,670    
Options to purchase shares of common stock outstanding 26,975    
Closing stock price $ 18.90    
Merger transaction costs $ 1,200    
Tocagen, Inc. | Restricted Stock Awards      
Business Acquisition [Line Items]      
Vested restricted stock awards outstanding 61,406