FAIR VALUE MEASUREMENT - Market Valuation Adjustments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Credit and funding valuation adjustments contra-liability (contra-asset)
Counterparty CVA	$ (617)		$ (617)		$ (800)
Asset FVA	(434)		(434)		(525)
Citigroup (own credit) CVA	335		335		403
Liability FVA	78		78		67
Total CVA—derivative instruments	(638)		(638)		$ (855)
Credit, Funding and Debt Valuation Adjustments Gain (Loss) [Abstract]
Counterparty CVA	34	$ 45	43	$ (238)
Asset FVA	25	632	94	(421)
Own credit CVA	(41)	(271)	(78)	262
Liability FVA	(13)	(214)	11	123
Total CVA—derivative instruments	5	192	70	(274)
DVA related to own FVO liabilities	(110)	(2,935)	(148)	1,253
Total CVA and DVA	$ (105)	$ (2,743)	$ (78)	$ 979