FAIR VALUE MEASUREMENT - Market Valuation Adjustments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Credit and funding valuation adjustments contra-liability (contra-asset)
Counterparty CVA	$ (815)		$ (815)		$ (970)
Asset FVA	(324)		(324)		(447)
Citigroup (own-credit) CVA	317		317		287
Liability FVA	39		39		47
Total CVA—derivative instruments	(783)		(783)		$ (1,083)
Credit, Funding and Debt Valuation Adjustments Gain (Loss) [Abstract]
Counterparty CVA	94	$ 27	117	$ 197
Asset FVA	74	(5)	123	74
Own-credit CVA	(75)	(2)	24	(127)
Liability FVA	(23)	(16)	(8)	(10)
Total CVA—derivative instruments	70	4	256	134
DVA related to own FVO liabilities	(377)	(195)	208	(422)
Total CVA and DVA	$ (307)	$ (191)	$ 464	$ (288)