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		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Numerator:
Net loss attributable to common stockholders of Cullinan	$ (24,804)	$ (17,350)	$ 137,996	$ (33,056)
Denominator
Weighted Average Number of Shares Outstanding, Basic	45,611	43,439	44,966	43,254
Dilutive effect of common stock issuable from assumed exercise of equity awards		0	1,614	0
Weighted Average Number of Shares Outstanding, Diluted	45,611	43,439	46,580	43,254
Earnings (net loss) per share
Earnings Per Share, Basic	$ (0.54)	$ (0.40)	$ 3.07	$ (0.76)
Earnings Per Share, Diluted	$ (0.54)	$ (0.40)	$ 2.96	$ (0.76)