Segment Reporting - Financial Information of Reportable Segment (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2025	Dec. 31, 2024
Segment Reporting Information [Line Items]
Research and development		$ 187,402	$ 142,903
Equity-based compensation		36,040	37,824
Loss from operations		(241,648)	(196,919)
Interest income		22,212	29,660
Other income (expense), net		(443)	(199)
Net loss before income taxes		(219,879)	(167,458)
Income tax expense (benefit)		0	117
Net loss		(219,879)	(167,575)
Net loss attributable to noncontrolling interests		(0)	(192)
Net loss attributable to Cullinan		(219,879)	(167,383)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Research and development		111,511	96,124
Equity-based compensation		36,040	37,824
R&D personnel and operations		39,986	31,532
G&A personnel and operations		13,572	14,578
License agreement obligations		20,303	100
Other segment expenses	[1]	20,236	16,761
Loss from operations		(241,648)	(196,919)
Interest income		22,212	29,660
Other income (expense), net		(443)	(199)
Net loss before income taxes		(219,879)	(167,458)
Income tax expense (benefit)		0	117
Net loss		(219,879)	(167,575)
Net loss attributable to noncontrolling interests		0	(192)
Net loss attributable to Cullinan		(219,879)	(167,383)
CLN-978 [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Research and development		23,074	14,833
CLN-619 [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Research and development		23,118	25,096
Zipalertinib [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Research and development		33,548	31,875
CLN-049 [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Research and development		17,088	7,508
CLN-617 [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Research and development		6,778	4,403
CLN-418 [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Research and development		0	6,471
Early-stage programs [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Research and development		6,286	5,938
Velinotamig [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Research and development		$ 1,619	$ 0

[1] Other segment expenses for 2025 and 2024 include legal fees relating to patent and corporate matters; professional fees for accounting, auditing, tax, and administrative consulting services; insurance costs; marketing expenses; depreciation; and other operating costs.