FAIR VALUE MEASUREMENT - Market Valuation Adjustments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Credit and funding valuation adjustments contra-liability (contra-asset)
Counterparty CVA	$ (1,715)		$ (1,715)		$ (1,853)
Asset FVA	(643)		(643)		(518)
Citigroup (own-credit) CVA	681		681		580
Liability FVA	108		108		19
Total CVA—derivative instruments	(1,569)		(1,569)		$ (1,772)
Credit, Funding and Debt Valuation Adjustments Gain (Loss) [Abstract]
Counterparty CVA	(32)	$ (24)	(191)	$ 46
Asset FVA	(177)	(480)	(125)	(480)
Own-credit CVA	97	15	81	(71)
Liability FVA	44	6	89	6
Total CVA—derivative instruments	(68)	(483)	(146)	(499)
DVA related to own FVO liabilities	264	112	582	102
Total CVA and DVA	$ 196	$ (371)	$ 436	$ (397)
Weighted average FICO score of the underlying collateral for Alt-A mortgage securities recorded at fair value, low end of range			680
Weighted average FICO score of the underlying collateral for Alt-A mortgage securities recorded at fair value, high end of range			720
Maximum percentage of underlying collateral where FICO scores are greater than 720			30.00%