Net Loss per Share - 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, 2022	Dec. 31, 2021
Numerator:
Net loss attributable to common stockholders of Cullinan	$ 111,214	$ (65,570)
Denominator
Weighted Average Number of Shares Outstanding, Basic	45,164	43,077
Dilutive effect of common stock issuable from assumed exercise of equity awards	1,476	0
Weighted Average Number of Shares Outstanding, Diluted	46,640	43,077
Earnings Per Share, Basic	$ 2.46	$ (1.52)
Earnings Per Share, Diluted	$ 2.38	$ (1.52)