CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Income Statement [Abstract]
Income (loss) from operations	$ (37,605)	$ 239,679	$ (100,361)	$ 207,022
Operating expenses:
Research and development	27,391	26,411	79,487	50,947
General and administrative	10,214	10,695	20,874	18,816
Total operating expenses	37,605	37,106	100,361	69,763
Gain on sale of Cullinan Pearl	0	276,785	0	276,785
Other income (expense):
Interest income	5,322	697	9,830	894
Other income (expense), net	69	(241)	176	241
Net income (loss) before income taxes	(32,214)	240,135	(90,355)	207,675
Income tax expense	0	66,070	0	46,502
Net income (loss)	(32,214)	174,065	(90,355)	161,173
Net loss attributable to noncontrolling interests	(0)	(833)	(179)	(1,627)
Net income (loss) attributable to common stockholders of Cullinan	(32,214)	174,898	(90,176)	162,800
Comprehensive income (loss):
Net income (loss)	(32,214)	174,065	(90,355)	161,173
Unrealized gain (loss) on investments	(383)	(499)	976	(2,795)
Comprehensive income (loss)	(32,597)	173,566	(89,379)	158,378
Comprehensive loss attributable to noncontrolling interests	(0)	(833)	(179)	(1,627)
Comprehensive income (loss) attributable to Cullinan	$ (32,597)	$ 174,399	$ (89,200)	$ 160,005
Net income (loss) per share:
Basic	$ (0.82)	$ 3.90	$ (2.24)	$ 3.65
Diluted	$ (0.82)	$ 3.77	$ (2.24)	$ 3.51
Weighted-average shares used in computing net income (loss) per share:
Weighted-average common stock outstanding - basic	39,952	44,873	40,315	44,654
Diluted	39,952	46,381	40,315	46,389