FAIR VALUE MEASUREMENT - Market Valuation Adjustments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Credit and funding valuation adjustments contra-liability (contra-asset)
Counterparty CVA	$ (1,666)		$ (1,666)		$ (1,853)
Asset FVA	(466)		(466)		(518)
Citigroup (own-credit) CVA	569		569		580
Liability FVA	64		64		19
Total CVA—derivative instruments	(1,499)		(1,499)		$ (1,772)
Credit, Funding and Debt Valuation Adjustments Gain (Loss) [Abstract]
Counterparty CVA	(20)	$ 63	(159)	$ 70
Asset FVA	94	0	52	0
Own-credit CVA	20	(52)	(16)	(86)
Liability FVA	(12)	0	45	0
Total CVA—derivative instruments	82	11	(78)	(16)
DVA related to own FVO liabilities	230	(44)	318	(10)
Total CVA and DVA	$ 312	$ (33)	$ 240	$ (26)
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%