FAIR VALUE MEASUREMENT - Market Valuation Adjustments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Credit and funding valuation adjustments contra-liability (contra-asset)
Counterparty CVA	$ (800)	$ (705)
Asset FVA	(525)	(530)
Citigroup (own-credit) CVA	403	341
Liability FVA	67	72
Total CVA—derivative instruments	(855)	(822)
Credit, Funding and Debt Valuation Adjustments Gain (Loss) [Abstract]
Counterparty CVA	(101)	149	$ (109)
Asset FVA	(95)	13	46
Own-credit CVA	133	(131)	178
Liability FVA	(6)	(63)	56
Total CVA—derivative instruments	(69)	(32)	171
DVA related to own FVO liabilities	(616)	(1,473)	1,415
Total CVA and DVA	$ (685)	$ (1,505)	$ 1,586