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INVESTMENTS - Carrying Value and Fair Value of HTM Debt Securities by Contractual Maturity Dates (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;    
Carrying value $ 23,254us-gaap_HeldToMaturitySecurities $ 23,921us-gaap_HeldToMaturitySecurities
Held-to-maturity Securities, Debt Maturities, Fair Value;    
Fair value 24,041us-gaap_HeldToMaturitySecuritiesFairValue 24,671us-gaap_HeldToMaturitySecuritiesFairValue
Akbank    
Held-to-maturity Securities, Debt Maturities, Fair Value;    
Equity investment ownership percentage   9.90%c_AvailableForSaleEquitySecuritiesOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= c_AkbankMember
Mortgage-backed securities    
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;    
Due within 1 year, carrying value 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember		 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember
After 1 but within 5 years, carrying value 7us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember		 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember
After 5 but within 10 years, carrying value 833us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember		 863us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember
After 10 years, carrying value 9,603us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember		 9,846us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember
Carrying value 10,443us-gaap_HeldToMaturitySecurities
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember		 10,709us-gaap_HeldToMaturitySecurities
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember
Held-to-maturity Securities, Debt Maturities, Fair Value;    
Due within 1 year, fair value 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember		 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember
After 1 but within 5 years, fair value 8us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember		 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember
After 5 but within 10 years, fair value 851us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember		 869us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember
After 10 years, fair value 10,125us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember		 10,303us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember
Fair value 10,984us-gaap_HeldToMaturitySecuritiesFairValue
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember		 11,172us-gaap_HeldToMaturitySecuritiesFairValue
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember
State and municipal securities    
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;    
Due within 1 year, carrying value 288us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 205us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
After 1 but within 5 years, carrying value 422us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 243us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
After 5 but within 10 years, carrying value 141us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 140us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
After 10 years, carrying value 7,077us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 7,361us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Carrying value 7,928us-gaap_HeldToMaturitySecurities
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 7,949us-gaap_HeldToMaturitySecurities
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Held-to-maturity Securities, Debt Maturities, Fair Value;    
Due within 1 year, fair value 281us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 205us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
After 1 but within 5 years, fair value 436us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 243us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
After 5 but within 10 years, fair value 148us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 144us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
After 10 years, fair value 7,204us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 7,527us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Fair value 8,069us-gaap_HeldToMaturitySecuritiesFairValue
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 8,119us-gaap_HeldToMaturitySecuritiesFairValue
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Foreign government    
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;    
Due within 1 year, carrying value 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
After 1 but within 5 years, carrying value 4,550us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		 4,725us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
After 5 but within 10 years, carrying value 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
After 10 years, carrying value 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Carrying value 4,550us-gaap_HeldToMaturitySecurities
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		 4,725us-gaap_HeldToMaturitySecurities
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Held-to-maturity Securities, Debt Maturities, Fair Value;    
Due within 1 year, fair value 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
After 1 but within 5 years, fair value 4,601us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		 4,802us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
After 5 but within 10 years, fair value 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
After 10 years, fair value 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Fair value 4,601us-gaap_HeldToMaturitySecuritiesFairValue
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		 4,802us-gaap_HeldToMaturitySecuritiesFairValue
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
All other    
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount;    
Due within 1 year, carrying value 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount
/ invest_InvestmentAxis
= c_CorporateDebtAssetBackedAndOtherDebtSecuritiesMember		 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearNetCarryingAmount
/ invest_InvestmentAxis
= c_CorporateDebtAssetBackedAndOtherDebtSecuritiesMember
After 1 but within 5 years, carrying value 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount
/ invest_InvestmentAxis
= c_CorporateDebtAssetBackedAndOtherDebtSecuritiesMember		 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount
/ invest_InvestmentAxis
= c_CorporateDebtAssetBackedAndOtherDebtSecuritiesMember
After 5 but within 10 years, carrying value 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount
/ invest_InvestmentAxis
= c_CorporateDebtAssetBackedAndOtherDebtSecuritiesMember		 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsNetCarryingAmount
/ invest_InvestmentAxis
= c_CorporateDebtAssetBackedAndOtherDebtSecuritiesMember
After 10 years, carrying value 333us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount
/ invest_InvestmentAxis
= c_CorporateDebtAssetBackedAndOtherDebtSecuritiesMember		 538us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount
/ invest_InvestmentAxis
= c_CorporateDebtAssetBackedAndOtherDebtSecuritiesMember
Carrying value 333us-gaap_HeldToMaturitySecurities
/ invest_InvestmentAxis
= c_CorporateDebtAssetBackedAndOtherDebtSecuritiesMember		 538us-gaap_HeldToMaturitySecurities
/ invest_InvestmentAxis
= c_CorporateDebtAssetBackedAndOtherDebtSecuritiesMember
Held-to-maturity Securities, Debt Maturities, Fair Value;    
Due within 1 year, fair value 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue
/ invest_InvestmentAxis
= c_CorporateDebtAssetBackedAndOtherDebtSecuritiesMember		 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue
/ invest_InvestmentAxis
= c_CorporateDebtAssetBackedAndOtherDebtSecuritiesMember
After 1 but within 5 years, fair value 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ invest_InvestmentAxis
= c_CorporateDebtAssetBackedAndOtherDebtSecuritiesMember		 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ invest_InvestmentAxis
= c_CorporateDebtAssetBackedAndOtherDebtSecuritiesMember
After 5 but within 10 years, fair value 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ invest_InvestmentAxis
= c_CorporateDebtAssetBackedAndOtherDebtSecuritiesMember		 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ invest_InvestmentAxis
= c_CorporateDebtAssetBackedAndOtherDebtSecuritiesMember
After 10 years, fair value 387us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue
/ invest_InvestmentAxis
= c_CorporateDebtAssetBackedAndOtherDebtSecuritiesMember		 578us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue
/ invest_InvestmentAxis
= c_CorporateDebtAssetBackedAndOtherDebtSecuritiesMember
Fair value $ 387us-gaap_HeldToMaturitySecuritiesFairValue
/ invest_InvestmentAxis
= c_CorporateDebtAssetBackedAndOtherDebtSecuritiesMember		 $ 578us-gaap_HeldToMaturitySecuritiesFairValue
/ invest_InvestmentAxis
= c_CorporateDebtAssetBackedAndOtherDebtSecuritiesMember