Net Loss per Share Attributable to Common Stockholders of Cullinan (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Numerator:
Net Income (Loss)	$ (37,148)	$ (57,962)
Denominator
Weighted-average common stock outstanding - basic	43,011	40,682
Weighted-average common stock outstanding - diluted	43,011	40,682
Net loss per share attributable to common stockholders of Cullinan:
Basic	$ (0.86)	$ (1.42)
Diluted	$ (0.86)	$ (1.42)