FAIR VALUE MEASUREMENT - Market Valuation Adjustments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Credit and funding valuation adjustments contra-liability (contra-asset)
Counterparty CVA	$ (1,243)		$ (1,243)		$ (705)
Asset FVA	(839)		(839)		(530)
Citigroup (own-credit) CVA	557		557		341
Liability FVA	195		195		72
Total CVA—derivative instruments	(1,330)		(1,330)		$ (822)
Credit, Funding and Debt Valuation Adjustments Gain (Loss) [Abstract]
Counterparty CVA	45	$ 28	(238)	$ 102
Asset FVA	632	(39)	(421)	(19)
Own-credit CVA	(271)	(13)	262	(105)
Liability FVA	(214)	18	123	(30)
Total CVA—derivative instruments	192	(6)	(274)	(52)
DVA related to own FVO liabilities	(2,935)	3	1,253	(722)
Total CVA and DVA	$ (2,743)	$ (3)	$ 979	$ (774)