CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	[1]
Income Statement [Abstract]
Revenue			$ 18,943
Type of revenue [Extensible List]	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember
Operating expenses:
Research and development	$ 11,778	$ 12,496	$ 24,193	$ 16,669
General and administrative	4,826	1,626	9,982	2,994
Total operating expenses	16,604	14,122	34,175	19,663
Loss from operations	(16,604)	(14,122)	(15,232)	(19,663)
Other income (expense):
Interest income	173	246	222	624
Other income (expense), net	(8)	1	(10)	1
Net loss	(16,439)	(13,875)	(15,020)	(19,038)
Net income/(loss) attributable to noncontrolling interest	(803)	(5,253)	686	(5,443)
Net loss attributable to common stockholders of Cullinan	$ (15,636)	$ (8,622)	$ (15,706)	$ (13,595)
Net loss per share, basic and diluted	$ (0.36)	$ (0.44)	$ (0.37)	$ (0.71)
Total weighted-average shares used in computing net loss per share, basic and diluted	43,295,372	19,619,748	42,713,059	19,115,380
Comprehensive loss:
Net loss	$ (16,439)	$ (13,875)	$ (15,020)	$ (19,038)
Unrealized gain/(loss) on investments	(55)	177	(113)	207
Comprehensive loss	(16,494)	(13,698)	(15,133)	(18,831)
Comprehensive income/(loss) attributable to noncontrolling interest	(803)	(5,253)	686	(5,443)
Comprehensive loss attributable to Cullinan	$ (15,691)	$ (8,445)	$ (15,819)	$ (13,388)

[1] The shares and per share amounts for the six months ended June 30, 2020 were derived from the unaudited consolidated financial statements as of that date and were retroactively adjusted as a result of the Reorganization and Reverse Stock Split. See Note 3 to the unaudited consolidated financial statements for additional details.