Segment Reporting - Financial Information of Reportable Segment (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Research and development		$ 142,903	$ 148,156
Equity-based compensation		37,824	30,438
Loss from operations		(196,919)	(191,089)
Interest income		29,660	21,627
Other income (expense), net		(199)	239
Net loss before income taxes		(167,458)	(169,223)
Income tax expense (benefit)		117	(14,122)
Net loss		(167,575)	(155,101)
Net loss attributable to noncontrolling interests		(192)	(1,939)
Net loss attributable to Cullinan		(167,383)	(153,162)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Research and development		96,124	84,783
Equity-based compensation		37,824	30,438
R&D personnel and operations		31,532	24,930
G&A personnel and operations		14,578	12,835
License agreement obligations		100	26,275
Other segment expenses	[1]	16,761	11,828
Loss from operations		(196,919)	(191,089)
Interest income		29,660	21,627
Other income (expense), net		(199)	239
Net loss before income taxes		(167,458)	(169,223)
Income tax expense (benefit)		117	(14,122)
Net loss		(167,575)	(155,101)
Net loss attributable to noncontrolling interests		(192)	(1,939)
Net loss attributable to Cullinan		(167,383)	(153,162)
CLN-978 [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Research and development		14,833	4,989
CLN-619 [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Research and development		25,096	18,667
Zipalertinib [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Research and development		31,875	31,230
CLN-049 [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Research and development		7,508	6,170
CLN-617 [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Research and development		4,403	5,016
CLN-418 [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Research and development		6,471	13,539
Early-stage programs [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Research and development		$ 5,938	$ 5,172

[1] Other segment expenses for 2024 and 2023 include legal fees relating to patent and corporate matters; professional fees for accounting, auditing, tax, and administrative consulting services; insurance costs; marketing expenses; impairment of long-lived assets; depreciation; and other operating costs.