DEBT - Long-Term Debt (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Debt Instrument
Long-term debt, at fair value	$ 258,274	$ 271,686
Trust preferred securities	1,700	1,700
Citigroup Inc.
Debt Instrument
Long-term debt, at fair value	170,104	170,563
Bank
Debt Instrument
Long-term debt, at fair value	24,715	44,742
Bank | Senior notes
Debt Instrument
Collateralized long-term advances from Federal Home Loan Bank	5,800	10,900
Broker-dealer and other
Debt Instrument
Long-term debt, at fair value	$ 63,455	$ 56,381