FAIR VALUE MEASUREMENT - Market Valuation Adjustments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Credit and funding valuation adjustments contra-liability (contra-asset)
Counterparty CVA	$ (1,085)	$ (970)
Asset FVA	(544)	(447)
Citigroup (own-credit) CVA	482	287
Liability FVA	135	47
Total CVA—derivative instruments	(1,012)	(1,083)
Credit, Funding and Debt Valuation Adjustments Gain (Loss) [Abstract]
Counterparty CVA	(109)	276	$ 157
Asset FVA	46	90	47
Own-credit CVA	178	(153)	17
Liability FVA	56	(15)	(44)
Total CVA—derivative instruments	171	198	177
DVA related to own FVO liabilities	1,415	(680)	(538)
Total CVA and DVA	$ 1,586	$ (482)	$ (361)