FAIR VALUE MEASUREMENT - Market Valuation Adjustments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Credit and funding valuation adjustments contra-liability (contra-asset)
Counterparty CVA	$ (866)		$ (866)		$ (1,085)
Asset FVA	(563)		(563)		(544)
Citigroup (own-credit) CVA	375		375		482
Liability FVA	105		105		135
Total CVA—derivative instruments	(949)		(949)		$ (1,012)
Credit, Funding and Debt Valuation Adjustments Gain (Loss) [Abstract]
Counterparty CVA	28	$ 0	102	$ 23
Asset FVA	(39)	40	(19)	49
Own-credit CVA	(13)	24	(105)	99
Liability FVA	18	22	(30)	15
Total CVA—derivative instruments	(6)	86	(52)	186
DVA related to own FVO liabilities	3	418	(722)	585
Total CVA and DVA	$ (3)	$ 504	$ (774)	$ 771