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, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021
Stockholders' Equity (Members' Deficit)
Common Stock	$ 4			$ 5
Additional paid-in capital	646,918			585,320
Accumulated other comprehensive loss	(1,051)			(2,601)
Accumulated deficit	(177,054)			(47,695)
Total Cullinan Stockholders' Equity (Members' Deficit)	468,817			535,029
Non-controlling interests in subsidiaries	0			0
Total Stockholders' Equity (Members' Deficit)	$ 468,817	$ 499,574	$ 485,649	$ 535,029	$ 586,128	$ 604,618	$ 419,470	$ 425,374