CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Income Statement [Abstract]
Revenue	$ 0	$ 0	$ 0	$ 18,943
Type of revenue [Extensible List]	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember
Operating expenses:
Research and development	$ 19,680	$ 12,680	$ 70,627	$ 36,873
General and administrative	10,086	5,695	28,902	15,677
Total operating expenses	29,766	18,375	99,529	52,550
Gain on sale of Cullinan Pearl	0	0	276,785	0
Income/(loss) from operations	(29,766)	(18,375)	177,256	(33,607)
Other income (expense):
Interest income	2,353	118	3,247	340
Other income (expense), net	0	(2)	(241)	(12)
Net income (loss) before income taxes	(27,413)	(18,259)	180,262	(33,279)
Income tax benefit	(2,523)	0	43,979	0
Net income (loss)	(24,890)	(18,259)	136,283	(33,279)
Net income (loss) attributable to noncontrolling interest	(86)	(909)	(1,713)	(223)
Net loss attributable to common stockholders of Cullinan	(24,804)	(17,350)	137,996	(33,056)
Comprehensive income (loss):
Net income (loss)	(24,890)	(18,259)	136,283	(33,279)
Unrealized gain (loss) on investments	(296)	57	(3,091)	(56)
Comprehensive income (loss)	(25,186)	(18,202)	133,192	(33,335)
Comprehensive income (loss) attributable to noncontrolling interest	(86)	(909)	(1,713)	(223)
Comprehensive income (loss) attributable to Cullinan	$ (25,100)	$ (17,293)	$ 134,905	$ (33,112)
Earnings (net loss) per share
Basic	$ (0.54)	$ (0.40)	$ 3.07	$ (0.76)
Diluted	$ (0.54)	$ (0.40)	$ 2.96	$ (0.76)
Weighted-average shares used in computing earnings (net loss) per share:
Basic	45,611	43,439	44,966	43,254
Diluted	45,611	43,439	46,580	43,254