FAIR VALUE MEASUREMENT - Market Valuation Adjustments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Credit and funding valuation adjustments contra-liability (contra-asset)
Counterparty CVA	$ (589)		$ (589)		$ (816)
Asset FVA	(539)		(539)		(622)
Citigroup (own credit) CVA	473		473		607
Liability FVA	273		273		263
Total CVA and FVA—derivative instruments	(382)		(382)		$ (568)
Credit, Funding and Debt Valuation Adjustments Gain (Loss) [Abstract]
Counterparty CVA	35	$ (10)	5	$ (211)
Asset FVA	(17)	(96)	77	(247)
Own credit CVA	14	29	(134)	327
Liability FVA	38	58	(5)	148
Total CVA and FVA—derivative instruments	70	(19)	(57)	17
DVA related to own FVO liabilities	395	1,159	(875)	4,800
Total CVA, DVA and FVA	$ 465	$ 1,140	$ (932)	$ 4,817