FAIR VALUE MEASUREMENT - Market Valuation Adjustments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Credit and funding valuation adjustments contra-liability (contra-asset)
Counterparty CVA	$ (1,862)		$ (1,853)
Asset FVA	(560)		(518)
Citigroup (own-credit) CVA	553		580
Liability FVA	75		19
Total CVA—derivative instruments	(1,794)		(1,772)
Credit, Funding and Debt Valuation Adjustments Gain (Loss) [Abstract]
Counterparty CVA	(139)	7
Asset FVA	(42)	0
Own-credit CVA	(36)	(34)
Liability FVA	57	0
Total CVA—derivative instruments	(160)	(27)
DVA related to own FVO liabilities	87	34
Total CVA and DVA	$ (73)	$ 7
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%

Counterparty Derivative Credit Risk Valuation Adjustment, Derivative Assets

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Counterparty Derivative Credit Risk Valuation Adjustment, Gain (Loss)

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- Definition

Derivative Credit and Debt Valuation Adjustment, Gain (Loss)

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Derivative Credit Risk Valuation Adjustment, Gain (Loss)

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- Definition

Derivative Debt Valuation Adjustment, Gain (Loss) on Fair Value Option Liability

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Derivative Funding Valuation Adjustment, Derivative Assets

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Derivative Funding Valuation Adjustment, Derivative Liabilities

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Derivative Funding Valuation Adjustment, Gain (Loss), Net

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Derivative Funding Valuation Adjustment, Gain (Loss)

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Derivative Funding Valuation Adjustment, Gain (Loss) on Derivative Liability

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- Definition

Maximum percentage of underlying collateral over residential mortgage-backed securities with full documentation loans where FICO scores are greater than 720.

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The high end weighted average FICO scores of underlying collateral of Alt-A non-agency residential mortgage-backed securities.

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The low end weighted average FICO scores of underlying collateral of Alt-A non-agency residential mortgage-backed securities.

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- Definition

Amount of credit risk valuation adjustment to derivative assets to properly reflect the credit quality of the counterparties.

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Amount of credit risk valuation adjustment to derivative liabilities to properly reflect the credit quality of the entity.

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