DEBT - Long-Term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Debt Instrument
Long-term debt, at fair value	$ 295,684	$ 287,300
Trust preferred securities	1,600	1,600
Citigroup Inc.
Debt Instrument
Long-term debt, at fair value	168,440	164,024
Bank
Debt Instrument
Long-term debt, at fair value	34,466	35,470
Bank | Senior notes
Debt Instrument
Collateralized long-term advances from Federal Home Loan Bank	7,500	8,500
Broker-dealer and other
Debt Instrument
Long-term debt, at fair value	$ 92,778	$ 87,806