FAIR VALUE MEASUREMENT - Market Valuation Adjustments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Credit and funding valuation adjustments contra-liability (contra-asset)
Counterparty CVA	$ (970)	$ (1,488)
Asset FVA	(447)	(536)
Citigroup (own-credit) CVA	287	459
Liability FVA	47	62
Total CVA—derivative instruments	(1,083)	(1,503)
Credit, Funding and Debt Valuation Adjustments Gain (Loss) [Abstract]
Counterparty CVA	276	157	$ (115)
Asset FVA	90	47	(66)
Own-credit CVA	(153)	17	(28)
Liability FVA	(15)	(44)	97
Total CVA—derivative instruments	198	177	(112)
DVA related to own FVO liabilities	(680)	(538)	367
Total CVA and DVA	$ (482)	$ (361)	$ 255