Earnings per Share - 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, 2023	Mar. 31, 2022
Numerator:
Net loss attributable to common stockholders of Cullinan	$ (57,962)	$ (12,098)
Denominator
Weighted-average common stock outstanding - basic	40,682	44,432
Weighted-average common stock outstanding - diluted	40,682	44,432
Net loss per share:
Basic	$ (1.42)	$ (0.27)
Diluted	$ (1.42)	$ (0.27)