DEBT - Long-Term Debt (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Debt Instrument
Long-term debt, at fair value	$ 275,760	$ 271,606
Trust preferred securities	1,600	1,600
Citigroup Inc.
Debt Instrument
Long-term debt, at fair value	160,571	166,257
Bank
Debt Instrument
Long-term debt, at fair value	24,560	21,113
Bank | Senior notes
Debt Instrument
Collateralized long-term advances from Federal Home Loan Bank	8,500	7,300
Broker-dealer and other
Debt Instrument
Long-term debt, at fair value	$ 90,629	$ 84,236