Net Loss per Share Attributable to Cullinan - Summary of Outstanding Equity Instruments Excluded from Computation of Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Equity instruments excluded from computation of diluted net loss per share (in shares)	21,061,000	15,762,000
Employee Stock Option
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Equity instruments excluded from computation of diluted net loss per share (in shares)	12,958,000	8,989,000
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)	6,455,000	6,475,000
Restricted stock awards 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)	1,644,000	292,000
Employee Stock Purchase Plan
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Equity instruments excluded from computation of diluted net loss per share (in shares)	4,000	6,000