FAIR VALUE MEASUREMENT - Market Valuation Adjustments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Credit and funding valuation adjustments contra-liability (contra-asset)
Counterparty CVA	$ (531)		$ (531)		$ (580)
Asset FVA	(478)		(478)		(562)
Citigroup (own credit) CVA	342		342		381
Liability FVA	213		213		255
Total CVA and FVA—derivative instruments	(454)		(454)		$ (506)
Credit, Funding and Debt Valuation Adjustments Gain (Loss) [Abstract]
Counterparty CVA	(38)	$ 35	(56)	$ 5
Asset FVA	1	(17)	87	77
Own credit CVA	(2)	14	(48)	(134)
Liability FVA	(13)	38	(42)	(5)
Derivative Credit Risk and Funding Valuation Adjustment, Gain (Loss), Net	(52)	70	(59)	(57)
DVA related to own FVO liabilities	(201)	395	(608)	(875)
Total CVA, DVA and FVA	$ (253)	$ 465	$ (667)	$ (932)