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		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Numerator:
Net income (loss) attributable to common stockholders of Cullinan	$ (39,183)	$ (24,804)	$ (129,359)	$ 137,996
Denominator
Weighted-average common stock outstanding - basic	42,734	45,611	41,130	44,966
Dilutive effect of common stock issuable from assumed exercise of equity awards	0	0	0	1,614
Weighted-average common stock outstanding - diluted	42,734	45,611	41,130	46,580
Net income (loss) per share attributable to common stockholders of Cullinan:
Basic	$ (0.91)	$ (0.54)	$ (3.15)	$ 3.07
Diluted	$ (0.91)	$ (0.54)	$ (3.15)	$ 2.96