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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	[1]
Numerator:
Net loss attributable to common stockholders of Cullinan	$ (17,350)	$ (9,858)	$ (33,056)	$ (23,453)
Denominator
Total weighted-average shares used in computing net loss per share, basic and diluted	43,438,861	19,721,992	43,254,230	19,453,479
Net Loss per Share:	$ (0.40)	$ (0.50)	$ (0.76)	$ (1.21)

[1] The shares and per share amounts for the nine months ended September 30, 2020 were derived from the unaudited consolidated financial statements as of that date and were retroactively adjusted as a result of the Reorganization and Reverse Stock Split. See Note 3 to the unaudited consolidated financial statements for additional details.