DEBT - Long-Term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Debt Instrument
Long-term debt, at fair value	$ 285,495	$ 286,619
Trust preferred securities	1,600	1,600
Citigroup Inc.
Debt Instrument
Long-term debt, at fair value	166,724	162,309
Bank
Debt Instrument
Long-term debt, at fair value	29,363	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	$ 89,408	$ 92,637