Net Income (Loss) per Share Attributable to Common Stockholders of Cullinan - Summary of Equity Instruments Excluded from Computation of Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Equity instruments excluded from computation of diluted net loss per share (in shares)	15,712	6,874
Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Equity instruments excluded from computation of diluted net loss per share (in shares)	9,418	6,842
Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Equity instruments excluded from computation of diluted net loss per share (in shares)	6,138	0
RSAs and RSUs
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Equity instruments excluded from computation of diluted net loss per share (in shares)	147	25
ESPP
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Equity instruments excluded from computation of diluted net loss per share (in shares)	9	7