CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Operating expenses:
Research and development	$ 61,030	$ 36,259	$ 102,489	$ 66,905
General and administrative	14,768	13,768	28,305	26,111
Total operating expenses	75,798	50,027	130,794	93,016
Loss from operations	(75,798)	(50,027)	(130,794)	(93,016)
Other income (expense):
Interest income	5,924	8,071	12,504	13,764
Other expense, net	(181)	(72)	(266)	(116)
Net loss	(70,055)	(42,028)	(118,556)	(79,368)
Net loss attributable to noncontrolling interests	0	0	0	(192)
Net loss attributable to Cullinan	(70,055)	(42,028)	(118,556)	(79,176)
Comprehensive income (loss):
Net loss	(70,055)	(42,028)	(118,556)	(79,368)
Unrealized gain (loss) on investments	102	(145)	740	(347)
Comprehensive loss	(69,953)	(42,173)	(117,816)	(79,715)
Comprehensive loss attributable to noncontrolling interests	0	0	0	(192)
Comprehensive loss attributable to Cullinan	(69,953)	(42,173)	(117,816)	(79,523)
Common Stock
Other income (expense):
Net loss attributable to Cullinan	$ (63,127)	$ (37,605)	$ (106,825)	$ (69,940)
Basic and diluted net loss per share attributable to Cullinan:
Net loss per share, basic	$ (1.07)	$ (0.68)	$ (1.81)	$ (1.43)
Net loss per share, diluted	$ (1.07)	$ (0.68)	$ (1.81)	$ (1.43)
Weighted-average shares used in computing basic and diluted net loss per share attributable to Cullinan:
Weighted-average shares outstanding - basic	59,015	55,052	58,960	49,031
Weighted-average shares outstanding - diluted	59,015	55,052	58,960	49,031
Preferred Stock
Other income (expense):
Net loss attributable to Cullinan	$ (6,928)	$ (4,423)	$ (11,731)	$ (9,236)
Basic and diluted net loss per share attributable to Cullinan:
Net loss per share, basic	$ (10.7)	$ (6.83)	$ (18.12)	$ (14.26)
Net loss per share, diluted	$ (10.7)	$ (6.83)	$ (18.12)	$ (14.26)
Weighted-average shares used in computing basic and diluted net loss per share attributable to Cullinan:
Weighted-average shares outstanding - basic	648	648	648	648
Weighted-average shares outstanding - diluted	648	648	648	648