Net Income (Loss) 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, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Numerator:
Net loss attributable to common stockholders of Cullinan	$ (32,214)	$ 174,898	$ (90,176)	$ 162,800
Denominator
Weighted-average common stock outstanding - basic	39,952	44,873	40,315	44,654
Dilutive effect of common stock issuable from assumed exercise of equity awards	0	1,508	0	1,735
Weighted-average common stock outstanding - diluted	39,952	46,381	40,315	46,389
Net income (loss) per share:
Basic	$ (0.82)	$ 3.90	$ (2.24)	$ 3.65
Diluted	$ (0.82)	$ 3.77	$ (2.24)	$ 3.51