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		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Numerator:
Net Income (Loss)	$ (42,028)	$ (32,214)	$ (79,176)	$ (90,176)
Denominator
Weighted-average common stock outstanding - basic	55,052	39,952	49,031	40,315
Weighted-average common stock outstanding - diluted	55,052	39,952	49,031	40,315
Net loss per share attributable to common stockholders of Cullinan:
Basic	$ (0.75)	$ (0.82)	$ (1.61)	$ (2.24)
Diluted	$ (0.75)	$ (0.82)	$ (1.61)	$ (2.24)