FAIR VALUE MEASUREMENT - Market Valuation Adjustments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Credit and funding valuation adjustments contra-liability (contra-asset)
Counterparty CVA	$ (671)		$ (671)		$ (800)
Asset FVA	(459)		(459)		(525)
Citigroup (own credit) CVA	375		375		403
Liability FVA	80		80		67
Total CVA—derivative instruments	(675)		(675)		$ (855)
Credit, Funding and Debt Valuation Adjustments Gain (Loss) [Abstract]
Counterparty CVA	25	$ 104	68	$ (134)
Asset FVA	(23)	105	71	(316)
Own credit CVA	34	(62)	(44)	200
Liability FVA	(63)	(36)	(52)	87
Total CVA—derivative instruments	(27)	111	43	(163)
DVA related to own FVO liabilities	(107)	(452)	(256)	801
Total CVA and DVA	$ (134)	$ (341)	$ (213)	$ 638