FAIR VALUE MEASUREMENT - Market Valuation Adjustments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Dec. 31, 2024
Credit and funding valuation adjustments contra-liability (contra-asset)
Counterparty CVA	$ (573)		$ (561)
Asset FVA	(537)		(539)
Citigroup (own credit) CVA	392		346
Liability FVA	215		209
Total CVA and FVA—derivative instruments	(503)		$ (545)
Credit, Funding and Debt Valuation Adjustments Gain (Loss) [Abstract]
Counterparty CVA	(24)	$ 8
Asset FVA	37	84
Own credit CVA	46	(52)
Liability FVA	5	(57)
Derivative Credit Risk and Funding Valuation Adjustment, Gain (Loss), Net	64	(17)
DVA related to own FVO liabilities	1,000	(750)
Total CVA, DVA and FVA	$ 1,064	$ (767)