Net Loss per Share Attributable to Cullinan - Summary of Calculation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Numerator:
Net loss attributable to Cullinan - basic and diluted	$ (219,879)	$ (167,383)
Common Stock
Numerator:
Net loss attributable to Cullinan - basic and diluted	$ (198,212)	$ (149,393)
Denominator
Weighted-average shares outstanding - basic	59,050	53,771
Weighted-average shares outstanding - diluted	59,050	53,771
Net loss per share attributable to Cullinan:
Basic	$ (3.36)	$ (2.78)
Diluted	$ (3.36)	$ (2.78)
Preferred Stock
Numerator:
Net loss attributable to Cullinan - basic and diluted	$ (21,667)	$ (17,990)
Denominator
Weighted-average shares outstanding - basic	645	648
Weighted-average shares outstanding - diluted	645	648
Net loss per share attributable to Cullinan:
Basic	$ (33.57)	$ (27.78)
Diluted	$ (33.57)	$ (27.78)