FAIR VALUE MEASUREMENT - Market Valuation Adjustments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Credit and funding valuation adjustments contra-liability (contra-asset)
Counterparty CVA	$ (982)		$ (1,085)
Asset FVA	(524)		(544)
Citigroup (own-credit) CVA	402		482
Liability FVA	87		135
Total CVA—derivative instruments	(1,017)		$ (1,012)
Credit, Funding and Debt Valuation Adjustments Gain (Loss) [Abstract]
Counterparty CVA	74	$ 23
Asset FVA	20	9
Own-credit CVA	(92)	75
Liability FVA	(48)	(7)
Total CVA—derivative instruments	(46)	100
DVA related to own FVO liabilities	(725)	167
Total CVA and DVA	$ (771)	$ 267