FAIR VALUE MEASUREMENT - Market Valuation Adjustments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Credit and funding valuation adjustments contra-liability (contra-asset)
Counterparty CVA	$ (1,114)		$ (1,114)		$ (1,488)
Asset FVA	(462)		(462)		(536)
Citigroup (own-credit) CVA	318		318		459
Liability FVA	51		51		62
Total CVA—derivative instruments	(1,207)		(1,207)		$ (1,503)
Credit, Funding and Debt Valuation Adjustments Gain (Loss) [Abstract]
Counterparty CVA	27	$ 112	197	$ 19
Asset FVA	(5)	37	74	(59)
Own-credit CVA	(2)	(60)	(127)	65
Liability FVA	(16)	(59)	(10)	(11)
Total CVA—derivative instruments	4	30	134	14
DVA related to own FVO liabilities	(195)	(319)	(422)	8
Total CVA and DVA	$ (191)	$ (289)	$ (288)	$ 22