Net Loss per Share - Summary of Calculation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Numerator:
Net loss attributable to common stockholders of Cullinan	$ (70)	$ (4,973)
Denominator
Total weighted-average shares used in computing net loss per share, basic and diluted	41,977,336	18,747,004
Net Loss per Share:	$ 0.00	$ (0.27)