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, 2025	Dec. 31, 2024	Mar. 31, 2024	Dec. 31, 2023
Stockholders' Equity (Members' Deficit)
Common Stock	$ 6	$ 6
Additional paid-in capital	968,069	958,695
Accumulated other comprehensive income (loss)	505	(133)
Accumulated deficit	(416,741)	(368,240)
Total Cullinan Stockholders' Equity (Members' Deficit)	551,839	590,328
Non-controlling interests in subsidiaries	0	0
Total Stockholders' Equity (Members' Deficit)	$ 551,839	$ 590,328	$ 425,665	$ 453,895