DEBT - Long-Term Debt (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Debt Instrument
Long-term debt, at fair value	$ 299,081	$ 286,619
Trust preferred securities	1,600	1,600
Citigroup Inc.
Debt Instrument
Long-term debt, at fair value	170,649	162,309
Bank
Debt Instrument
Long-term debt, at fair value	36,548	31,673
Bank | Senior notes
Debt Instrument
Collateralized long-term advances from Federal Home Loan Bank	11,500	11,500
Broker-dealer and other
Debt Instrument
Long-term debt, at fair value	$ 91,884	$ 92,637