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	Mar. 31, 2021	Dec. 31, 2020		Mar. 31, 2020	Dec. 31, 2019
Stockholders' Equity (Members' Deficit)
Common Stock	$ 4	$ 3	[1]		$ 2
Additional paid-in capital	560,366	292,348	[1]		138,543
Accumulated other comprehensive loss	(60)	(2)	[1]		(4)
Accumulated deficit	(93,409)	(93,339)	[1]		(41,540)
Total Cullinan Stockholders' Equity (Members' Deficit)	466,901	199,010	[1]		97,001
Non-controlling interests in subsidiaries	2,798	1,304	[1]		864
Total Stockholders' Equity (Members' Deficit)	$ 469,699	200,314	[1],[2]	$ 106,769	97,865	[2]
As Reported
Redeemable Preferred Units
Total Redeemable Preferred Units		276,652			137,774
Stockholders' Equity (Members' Deficit)
Non-Voting Incentive Units		1			1
Additional paid-in capital		15,698			770
Accumulated other comprehensive loss		(2)			(4)
Accumulated deficit		(93,339)			(41,540)
Total Cullinan Stockholders' Equity (Members' Deficit)		(77,642)			(40,773)
Non-controlling interests in subsidiaries		1,304			864
Total Stockholders' Equity (Members' Deficit)		(76,338)			(39,909)
Adjustment
Redeemable Preferred Units
Total Redeemable Preferred Units		(276,652)			(137,774)
Stockholders' Equity (Members' Deficit)
Non-Voting Incentive Units		(1)			(1)
Common Stock		3			2
Additional paid-in capital		276,650			137,773
Total Cullinan Stockholders' Equity (Members' Deficit)		276,652			137,774
Total Stockholders' Equity (Members' Deficit)		276,652			137,774
Series Seed Redeemable Preferred Units | As Reported
Redeemable Preferred Units
Total Redeemable Preferred Units		3,956			3,956
Series Seed Redeemable Preferred Units | Adjustment
Redeemable Preferred Units
Total Redeemable Preferred Units		(3,956)			(3,956)
Series A1 Redeemable Preferred Units | As Reported
Redeemable Preferred Units
Total Redeemable Preferred Units		49,946			49,946
Series A1 Redeemable Preferred Units | Adjustment
Redeemable Preferred Units
Total Redeemable Preferred Units		(49,946)			(49,946)
Series B Redeemable Preferred Units | As Reported
Redeemable Preferred Units
Total Redeemable Preferred Units		97,909			83,872
Series B Redeemable Preferred Units | Adjustment
Redeemable Preferred Units
Total Redeemable Preferred Units		(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 consolidated financial statements for additional details.

[2] The consolidated balance sheets as of December 31, 2019 and 2020 are derived from the audited consolidated financial statements as of those dates and were 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 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 consolidated financial statements for additional details.