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Organization and Description of Business - Additional Information (Details)
$ in Thousands
6 Months Ended
Jun. 30, 2021
USD ($)
shares
Jun. 30, 2020
USD ($)
Dec. 31, 2020
USD ($)
Organization Consolidation And Presentation Of Financial Statements [Line Items]      
Accumulated deficit $ 62,073   $ 51,457
Cash used in operating activities (7,968) $ (6,881)  
Cash and cash equivalents $ 50,795   $ 58,765
Tocagen, Inc.      
Organization Consolidation And Presentation Of Financial Statements [Line Items]      
Right to receive shares for each common stock outstanding | shares 3.1624    
Reverse stock split, description 1‑for‑15    
Reverse split ratio 0.06667    
Common stock ownership percentage 15.30%    
Forte Subsidiary, Inc.      
Organization Consolidation And Presentation Of Financial Statements [Line Items]      
Common stock ownership percentage 84.70%