Net Loss per Share Attributable to Cullinan - Summary of Outstanding Equity Instruments Excluded from Computation of Diluted Net Loss Per Share (Details) - shares shares in Thousands	6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Shares excluded from computation of diluted net loss per share (in shares)	20,695	15,072
Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Shares excluded from computation of diluted net loss per share (in shares)	12,601	8,289
Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Shares excluded from computation of diluted net loss per share (in shares)	6,475	6,475
Restricted Stock Awards and RSUs
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Shares excluded from computation of diluted net loss per share (in shares)	1,609	301
Employee Stock Purchase Plan
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Shares excluded from computation of diluted net loss per share (in shares)	10	7