FAIR VALUE MEASUREMENT - Market Valuation Adjustments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Credit and funding valuation adjustments contra-liability (contra-asset)
Counterparty CVA	$ (519)		$ (519)		$ (580)
Asset FVA	(466)		(466)		(562)
Citigroup (own credit) CVA	345		345		381
Liability FVA	226		226		255
Total CVA and FVA—derivative instruments	(414)		(414)		$ (506)
Credit, Funding and Debt Valuation Adjustments Gain (Loss) [Abstract]
Counterparty CVA	(25)	$ 4	(17)	$ (30)
Asset FVA	2	100	86	94
Own credit CVA	6	(114)	(46)	(149)
Liability FVA	27	(17)	(30)	(44)
Derivative Credit Risk and Funding Valuation Adjustment, Gain (Loss), Net	10	(27)	(7)	(129)
DVA related to own FVO liabilities	343	(837)	(407)	(1,270)
Total CVA, DVA and FVA	$ 353	$ (864)	$ (414)	$ (1,399)