Reorganization and Reverse Stock Split - Number of Shares of Common Stock Exchanged and Issued to Unitholders on Split Adjusted Basis (Details) - shares	1 Months Ended	12 Months Ended
	Jan. 31, 2021	Dec. 31, 2021
Reorganization And Reverse Stock Split [Line Items]
Number of Management's Shares Issued	320,228
Cullinan Oncology, LLC
Reorganization And Reverse Stock Split [Line Items]
Number of Management's Shares Issued		242,384,283
Adjusted for the Reverse Stock Split		34,434,262
Series C Preferred Units | Cullinan Oncology, LLC
Reorganization And Reverse Stock Split [Line Items]
Number of Management's Shares Issued		66,599,045
Adjusted for the Reverse Stock Split		9,461,414
Series B Preferred Units | Cullinan Oncology, LLC
Reorganization And Reverse Stock Split [Line Items]
Number of Management's Shares Issued		63,141,016
Adjusted for the Reverse Stock Split		8,970,154
Series A1 Preferred Units | Cullinan Oncology, LLC
Reorganization And Reverse Stock Split [Line Items]
Number of Management's Shares Issued		50,000,000
Adjusted for the Reverse Stock Split		7,103,280
Series Seed Preferred Units | Cullinan Oncology, LLC
Reorganization And Reverse Stock Split [Line Items]
Number of Management's Shares Issued		16,000,000
Adjusted for the Reverse Stock Split		2,273,050
Non-Voting Incentive Units | Cullinan Oncology, LLC
Reorganization And Reverse Stock Split [Line Items]
Number of Management's Shares Issued		11,896,500
Adjusted for the Reverse Stock Split		1,689,949
Common Units | Cullinan Oncology, LLC
Reorganization And Reverse Stock Split [Line Items]
Number of Management's Shares Issued		34,747,722
Adjusted for the Reverse Stock Split		4,936,415