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	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 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	564,705		292,348	[1]	152,580		138,543
Accumulated other comprehensive loss	(115)		(2)	[1]	203		(4)
Accumulated deficit	(109,045)		(93,339)	[1]	(55,135)		(41,540)
Total Cullinan Stockholders' Equity (Members' Deficit)	455,549		199,010	[1]	97,650		97,001
Non-controlling interests in subsidiaries	2,991		1,304	[1]	1,269		864
Total Stockholders' Equity (Members' Deficit)	$ 458,540	$ 469,699	200,314	[1],[2]	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)		203		(4)
Accumulated deficit			(93,339)		(55,135)		(41,540)
Total Cullinan Stockholders' Equity (Members' Deficit)			(77,642)		(54,161)		(40,773)
Non-controlling interests in subsidiaries			1,304		1,269		864
Total Stockholders' Equity (Members' Deficit)			(76,338)		(52,892)		(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 and June 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.