CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	[1]
Income Statement [Abstract]
Revenue			$ 18,943
Type of revenue [Extensible List]	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember
Operating expenses:
Research and development	$ 12,680	$ 9,913	$ 36,873	$ 26,582
General and administrative	5,695	1,586	15,677	4,580
Total operating expenses	18,375	11,499	52,550	31,162
Loss from operations	(18,375)	(11,499)	(33,607)	(31,162)
Other income (expense):
Interest income	118	185	340	809
Other income (expense), net	(2)		(12)	1
Net loss	(18,259)	(11,314)	(33,279)	(30,352)
Net loss attributable to noncontrolling interest	(909)	(1,456)	(223)	(6,899)
Net loss attributable to common stockholders of Cullinan	$ (17,350)	$ (9,858)	$ (33,056)	$ (23,453)
Net loss per share, basic and diluted	$ (0.40)	$ (0.50)	$ (0.76)	$ (1.21)
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
Comprehensive loss:
Net loss	$ (18,259)	$ (11,314)	$ (33,279)	$ (30,352)
Unrealized gain/(loss) on investments	57	(144)	(56)	63
Comprehensive loss	(18,202)	(11,458)	(33,335)	(30,289)
Comprehensive loss attributable to noncontrolling interest	(909)	(1,456)	(223)	(6,899)
Comprehensive loss attributable to Cullinan	$ (17,293)	$ (10,002)	$ (33,112)	$ (23,390)

[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.