Net Loss per Share Attributable to Common Stockholders of Cullinan - Summary of Calculation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Numerator:
Net loss attributable to common stockholders of Cullinan	$ (48,501)	$ (37,148)
Common Stock
Numerator:
Net loss attributable to common stockholders of Cullinan	$ (43,698)	$ (32,287)
Denominator
Weighted-average common stock outstanding - basic	58,905	43,011
Weighted-average common stock outstanding - diluted	58,905	43,011
Net loss per share attributable to common stockholders of Cullinan:
Basic	$ (0.74)	$ (0.75)
Diluted	$ (0.74)	$ (0.75)
Preferred Stock
Numerator:
Net loss attributable to common stockholders of Cullinan	$ (4,803)	$ (4,861)
Denominator
Weighted-average common stock outstanding - basic	648	648
Weighted-average common stock outstanding - diluted	648	648
Net loss per share attributable to common stockholders of Cullinan:
Basic	$ (7.42)	$ (7.51)
Diluted	$ (7.42)	$ (7.51)