DEBT - Long-Term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Debt Instrument
Long-term debt, at fair value	$ 264,575	$ 271,686
Trust preferred securities	1,700	1,700
Citigroup Inc.
Debt Instrument
Long-term debt, at fair value	174,366	170,563
Bank
Debt Instrument
Long-term debt, at fair value	28,661	44,742
Bank | Senior notes
Debt Instrument
Collateralized long-term advances from Federal Home Loan Bank	9,500	10,900
Broker-dealer and other
Debt Instrument
Long-term debt, at fair value	$ 61,548	$ 56,381