Net Loss per Share Attributable to Cullinan (Additional Information) (Details) - $ / shares	3 Months Ended			6 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2024	Sep. 30, 2024	Dec. 31, 2024	Dec. 31, 2023
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	$ (0.63)	$ (0.68)	$ (0.75)	$ (1.43)	$ (2.04)	$ (2.78)	$ (3.21)
Net loss per share attributable to common stockholders of Cullinan, diluted	(0.63)	(0.68)	(0.75)	(1.43)	(2.04)	(2.78)	(3.21)
Common Stock [Member] | Previously presented
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net loss per share attributable to common stockholders of Cullinan, basic	(0.69)	(0.75)	(0.86)	(1.61)	(2.3)		(3.69)
Net loss per share attributable to common stockholders of Cullinan, diluted	(0.69)	(0.75)	(0.86)	(1.61)	(2.3)		(3.69)
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	(6.26)	(6.83)	(7.51)	(14.26)	(20.42)	(27.78)	(32.12)
Net loss per share attributable to common stockholders of Cullinan, diluted	$ (6.26)	$ (6.83)	$ (7.51)	$ (14.26)	$ (20.42)	$ (27.78)	$ (32.12)