FAIR VALUE MEASUREMENT - Market Valuation Adjustments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Credit and funding valuation adjustments contra-liability (contra-asset)
Counterparty CVA	$ (1,889)		$ (1,470)
Asset FVA	(664)		(584)
Citigroup (own-credit) CVA	609		471
Liability FVA	135		106
Total CVA—derivative instruments	(1,809)		$ (1,477)
Credit, Funding and Debt Valuation Adjustments Gain (Loss) [Abstract]
Counterparty CVA	(108)	$ (139)
Asset FVA	(80)	(42)
Own-credit CVA	135	(36)
Liability FVA	29	57
Total CVA—derivative instruments(1)	(24)	(160)
DVA related to own FVO liabilities	$ 307	$ 87
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%