FAIR VALUE MEASUREMENT - Market Valuation Adjustments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Credit and funding valuation adjustments contra-liability (contra-asset)
Counterparty CVA	$ (486)		$ (580)
Asset FVA	(463)		(562)
Citigroup (own credit) CVA	327		381
Liability FVA	199		255
Total CVA and FVA—derivative instruments	(423)		$ (506)
Credit, Funding and Debt Valuation Adjustments Gain (Loss) [Abstract]
Counterparty CVA	8	$ (34)
Asset FVA	84	(6)
Own credit CVA	(52)	(35)
Liability FVA	(57)	(28)
Derivative Credit Risk and Funding Valuation Adjustment, Gain (Loss), Net	(17)	(103)
DVA related to own FVO liabilities	(750)	(433)
Total CVA, DVA and FVA	$ (767)	$ (536)