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		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Numerator:
Net loss attributable to common stockholders of Cullinan	$ 174,898	$ (15,636)	$ 162,800	$ (15,706)
Denominator
Weighted Average Number of Shares Outstanding, Basic	44,873	43,295	44,654	42,713
Dilutive effect of common stock issuable from assumed exercise of equity awards	1,508	0	1,735	0
Weighted Average Number of Shares Outstanding, Diluted	46,381	43,295	46,389	42,713
Earnings (net loss) per share
Earnings Per Share, Basic	$ 3.90	$ (0.36)	$ 3.65	$ (0.37)
Earnings Per Share, Diluted	$ 3.77	$ (0.36)	$ 3.51	$ (0.37)