Net Loss per Share Attributable to Cullinan - Summary of Calculation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended			6 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2024	Sep. 30, 2024	Dec. 31, 2024	Dec. 31, 2023
Numerator:
Net loss attributable to Cullinan - basic and diluted						$ (167,383)	$ (153,162)
Common Stock
Numerator:
Net loss attributable to Cullinan - basic and diluted						$ (149,393)	$ (133,448)
Net loss attributable to Cullinan - basic and diluted	$ (36,508)	$ (37,605)	$ (32,287)	$ (69,940)	$ (106,513)
Denominator
Weighted-average shares outstanding - basic	58,337	55,052	43,011	49,031	52,157	53,771	41,550
Weighted-average shares outstanding - diluted	58,337	55,052	43,011	49,031	52,157	53,771	41,550
Net loss per share attributable to Cullinan:
Basic	$ (0.63)	$ (0.68)	$ (0.75)	$ (1.43)	$ (2.04)	$ (2.78)	$ (3.21)
Diluted	$ (0.63)	$ (0.68)	$ (0.75)	$ (1.43)	$ (2.04)	$ (2.78)	$ (3.21)
Preferred Stock
Numerator:
Net loss attributable to Cullinan - basic and diluted						$ (17,990)	$ (19,714)
Net loss attributable to Cullinan - basic and diluted	$ (4,052)	$ (4,423)	$ (4,861)	$ (9,236)	$ (13,223)
Denominator
Weighted-average shares outstanding - basic	648	648	648	648	648	648	614
Weighted-average shares outstanding - diluted	648	648	648	648	648	648	614
Net loss per share attributable to Cullinan:
Basic	$ (6.26)	$ (6.83)	$ (7.51)	$ (14.26)	$ (20.42)	$ (27.78)	$ (32.12)
Diluted	$ (6.26)	$ (6.83)	$ (7.51)	$ (14.26)	$ (20.42)	$ (27.78)	$ (32.12)