Organization and Description of Business - Additional Information (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021 USD ($) shares	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Accumulated deficit	$ 69,824		$ 51,457
Cash used in operating activities	(13,095)	$ (14,620)
Cash and cash equivalents	$ 45,665		$ 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%