Net Income (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	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Numerator:
Net income (loss) attributable to common stockholders of Cullinan	$ (153,162)	$ 111,214
Denominator
Weighted-average common stock outstanding - basic	41,550	45,164
Dilutive effect of common stock issuable from assumed exercise of equity awards	0	1,476
Weighted-average common stock outstanding - diluted	41,550	46,640
Basic	$ (3.69)	$ 2.46
Diluted	$ (3.69)	$ 2.38