CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Income Statement [Abstract]
Income (loss) from operations	$ (45,243)	$ (29,766)	$ (145,604)	$ 177,256
Operating expenses:
Research and development	33,821	19,680	113,308	70,627
General and administrative	10,982	10,086	31,856	28,902
Total operating expenses	44,803	29,766	145,164	99,529
Gain on sale of Cullinan Pearl	0	0	0	276,785
Impairment of long-lived assets	(440)	0	(440)	0
Other income (expense):
Interest income	5,880	2,353	15,710	3,247
Other income (expense), net	180	0	356	(241)
Net income (loss) before income taxes	(39,183)	(27,413)	(129,538)	180,262
Income tax expense (benefit)	0	(2,523)	0	43,979
Net income (loss)	(39,183)	(24,890)	(129,538)	136,283
Net loss attributable to noncontrolling interests	0	(86)	(179)	(1,713)
Net income (loss) attributable to common stockholders of Cullinan	(39,183)	(24,804)	(129,359)	137,996
Comprehensive income (loss):
Net income (loss)	(39,183)	(24,890)	(129,538)	136,283
Unrealized gain on investments	574	(296)	1,550	(3,091)
Comprehensive income (loss)	(38,609)	(25,186)	(127,988)	133,192
Comprehensive loss attributable to noncontrolling interests	0	(86)	(179)	(1,713)
Comprehensive income (loss) attributable to Cullinan	$ (38,609)	$ (25,100)	$ (127,809)	$ 134,905
Net income (loss) per share attributable to common stockholders of Cullinan:
Basic	$ (0.91)	$ (0.54)	$ (3.15)	$ 3.07
Diluted	$ (0.91)	$ (0.54)	$ (3.15)	$ 2.96
Weighted-average shares used in computing net income (loss) per share attributable to common stockholders of Cullinan
Weighted-average common stock outstanding - basic	42,734	45,611	41,130	44,966
Diluted	42,734	45,611	41,130	46,580