CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 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	$ 26,411	$ 11,778	$ 50,947	$ 24,193
General and administrative	10,695	4,826	18,816	9,982
Total operating expenses	37,106	16,604	69,763	34,175
Gain on sale of Cullinan Pearl	276,785	0	276,785	0
Income/(loss) from operations	239,679	(16,604)	207,022	(15,232)
Other income (expense):
Interest income	697	173	894	222
Other income (expense), net	(241)	(8)	(241)	(10)
Net income/(loss) before income taxes	240,135	(16,439)	207,675	(15,020)
Income tax expense	66,070	0	46,502	0
Net income/(loss)	174,065	(16,439)	161,173	(15,020)
Net income/(loss) attributable to noncontrolling interest	(833)	(803)	(1,627)	686
Net loss attributable to common stockholders of Cullinan	174,898	(15,636)	162,800	(15,706)
Comprehensive income/(loss):
Net income (loss)	174,065	(16,439)	161,173	(15,020)
Unrealized gain/(loss) on investments	(499)	(55)	(2,795)	(113)
Comprehensive income/(loss)	173,566	(16,494)	158,378	(15,133)
Comprehensive income/(loss) attributable to noncontrolling interest	(833)	(803)	(1,627)	686
Comprehensive income/(loss) attributable to Cullinan	$ 174,399	$ (15,691)	$ 160,005	$ (15,819)
Earnings (net loss) per share
Basic	$ 3.90	$ (0.36)	$ 3.65	$ (0.37)
Diluted	$ 3.77	$ (0.36)	$ 3.51	$ (0.37)
Weighted-average shares used in computing earnings (net loss) per share:
Basic	44,873	43,295	44,654	42,713
Diluted	46,381	43,295	46,389	42,713