Reorganization and Reverse Stock Split - Summary of LLC Units Previously Reported as Redeemable Preferred Units or Temporary Equity Converted Reclassified to Equity (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020		Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Stockholders' Equity (Members' Deficit)
Common Stock	$ 4			$ 3	[1]	$ 2			$ 2
Additional paid-in capital	569,835			292,348	[1]	152,580			138,543
Accumulated other comprehensive loss	(58)			(2)	[1]	59			(4)
Accumulated deficit	(126,395)			(93,339)	[1]	(64,993)			(41,540)
Total Cullinan Stockholders' Equity (Members' Deficit)	443,386			199,010	[1]	87,648			97,001
Non-controlling interests in subsidiaries	2,088			1,304	[1]	1,863			864
Total Stockholders' Equity (Members' Deficit)	$ 445,474	$ 458,540	$ 469,699	200,314	[1],[2]	89,511	$ 98,919	$ 106,769	97,865	[3]
As Reported
Redeemable Preferred Units
Total Redeemable Preferred Units				276,652		151,811			137,774
Stockholders' Equity (Members' Deficit)
Non-Voting Incentive Units				1		1			1
Additional paid-in capital				15,698		770			770
Accumulated other comprehensive loss				(2)		59			(4)
Accumulated deficit				(93,339)		(64,993)			(41,540)
Total Cullinan Stockholders' Equity (Members' Deficit)				(77,642)		(64,163)			(40,773)
Non-controlling interests in subsidiaries				1,304		1,863			864
Total Stockholders' Equity (Members' Deficit)				(76,338)		(62,300)			(39,909)
Adjustment
Redeemable Preferred Units
Total Redeemable Preferred Units				(276,652)		(151,811)			(137,774)
Stockholders' Equity (Members' Deficit)
Non-Voting Incentive Units				(1)		(1)			(1)
Common Stock				3		2			2
Additional paid-in capital				276,650		151,810			137,773
Total Cullinan Stockholders' Equity (Members' Deficit)				276,652		151,811			137,774
Total Stockholders' Equity (Members' Deficit)				276,652		151,811			137,774
Series Seed Redeemable Preferred Units | As Reported
Redeemable Preferred Units
Total Redeemable Preferred Units				3,956		3,956			3,956
Series Seed Redeemable Preferred Units | Adjustment
Redeemable Preferred Units
Total Redeemable Preferred Units				(3,956)		(3,956)			(3,956)
Series A1 Redeemable Preferred Units | As Reported
Redeemable Preferred Units
Total Redeemable Preferred Units				49,946		49,946			49,946
Series A1 Redeemable Preferred Units | Adjustment
Redeemable Preferred Units
Total Redeemable Preferred Units				(49,946)		(49,946)			(49,946)
Series B Redeemable Preferred Units | As Reported
Redeemable Preferred Units
Total Redeemable Preferred Units				97,909		97,909			83,872
Series B Redeemable Preferred Units | Adjustment
Redeemable Preferred Units
Total Redeemable Preferred Units				(97,909)		$ (97,909)			$ (83,872)
Series C Redeemable Preferred Units | As Reported
Redeemable Preferred Units
Total Redeemable Preferred Units				124,841
Series C Redeemable Preferred Units | Adjustment
Redeemable Preferred Units
Total Redeemable Preferred Units				$ (124,841)

[1] The consolidated balance sheet as of December 31, 2020 is derived from the audited consolidated financial statements as of that date and was retroactively adjusted, including shares and per share amounts, as a result of the Reorganization and Reverse Stock Split. See Note 3 to the unaudited consolidated financial statements for additional details.

[2] The consolidated balance sheet as of December 31, 2020 is derived from the audited consolidated financial statements as of such date and was retroactively adjusted, including shares and per share amounts, as a result of the Reorganization and Reverse Stock Split. See Note 3 to the unaudited consolidated financial statements for additional details.

[3] The consolidated balance sheet as of December 31, 2019 is derived from the audited consolidated financial statements as of such date and was retroactively adjusted, including shares and per share amounts, as a result of the Reorganization and Reverse Stock Split. Changes in stockholders’ equity for the three months ended March 31, 2020, June 30, 2020 and September 30, 2020 were also retroactively adjusted, including shares and per share amounts, as a result of the Reorganization and Reverse Stock Split. See Note 3 to the unaudited consolidated financial statements for additional details.