Net Loss per Share Attributable to Cullinan (Additional Information) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net loss per share attributable to common stockholders of Cullinan, basic	$ (3.36)	$ (2.78)
Net loss per share attributable to common stockholders of Cullinan, diluted	(3.36)	(2.78)
Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net loss per share attributable to common stockholders of Cullinan, basic	(33.57)	(27.78)
Net loss per share attributable to common stockholders of Cullinan, diluted	$ (33.57)	$ (27.78)