Net Loss per Share Attributable to Common Stockholders of Cullinan - Summary of Outstanding Equity Instruments Excluded from Computation of Diluted Net Loss Per Share (Details) - shares shares in Thousands	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Shares excluded from computation of diluted net loss per share (in shares)	15,223	15,538
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)	8,411	9,355
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	6,024
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)	328	158
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)	9	1