FAIR VALUE MEASUREMENT - Market Valuation Adjustments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Credit and funding valuation adjustments contra-liability (contra-asset)
Counterparty CVA	$ (565)		$ (565)		$ (561)
Asset FVA	(572)		(572)		(539)
Citigroup (own credit) CVA	366		366		346
Liability FVA	187		187		209
Total CVA and FVA—derivative instruments	(584)		(584)		$ (545)
Credit, Funding and Debt Valuation Adjustments Gain (Loss) [Abstract]
Counterparty CVA	2	$ (38)	(59)	$ (56)
Asset FVA	27	1	24	87
Own credit CVA	(4)	(2)	15	(48)
Liability FVA	(25)	(13)	(8)	(42)
Derivative Credit Risk and Funding Valuation Adjustment, Gain (Loss), Net	0	(52)	(28)	(59)
DVA related to own FVO liabilities	(1,293)	(201)	(684)	(608)
Total CVA, DVA and FVA	$ (1,293)	$ (253)	$ (712)	$ (667)