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INVESTMENTS - Overview (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Investment Holdings      
Investments $ 326,815c_InvestmentsExcludingTradingSecurities   $ 333,443c_InvestmentsExcludingTradingSecurities
Interest and dividends on investments      
Taxable interest 1,593us-gaap_InterestIncomeSecuritiesTaxable 1,467us-gaap_InterestIncomeSecuritiesTaxable  
Interest exempt from U.S. federal income tax 23us-gaap_InterestIncomeSecuritiesTaxExempt 164us-gaap_InterestIncomeSecuritiesTaxExempt  
Dividends 95us-gaap_DividendIncomeOperating 126us-gaap_DividendIncomeOperating  
Total interest and dividends 1,711c_InterestAndDividendIncomeSecuritiesExcludingTradingSecurities 1,757c_InterestAndDividendIncomeSecuritiesExcludingTradingSecurities  
Gross realized investments losses, excluding losses from other-than-temporary impairment      
Gross realized investment gains 356c_GrossRealizedGainOnMarketableSecuritiesAndCostMethodInvestmentsExcludingOtherThanTemporaryImpairmentsAndOtherInvestments 292c_GrossRealizedGainOnMarketableSecuritiesAndCostMethodInvestmentsExcludingOtherThanTemporaryImpairmentsAndOtherInvestments  
Gross realized investment losses (49)c_GrossRealizedLossonMarketableSecuritiesandCostMethodInvestmentsExcludingOtherthanTemporaryImpairmentsandOtherInvestments (164)c_GrossRealizedLossonMarketableSecuritiesandCostMethodInvestmentsExcludingOtherthanTemporaryImpairmentsandOtherInvestments  
Net realized gains (losses) 307us-gaap_RealizedGainLossOnMarketableSecuritiesAndCostMethodInvestmentsExcludingOtherThanTemporaryImpairmentsAndOtherInvestments [1] 128us-gaap_RealizedGainLossOnMarketableSecuritiesAndCostMethodInvestmentsExcludingOtherThanTemporaryImpairmentsAndOtherInvestments [1]  
Available-for-sale Securities transferred from Held-to-maturity      
Carrying value of HTM securities sold 27c_HeldtomaturitySecuritiesSoldSecurityCarryingValue 0c_HeldtomaturitySecuritiesSoldSecurityCarryingValue  
Net realized gain (loss) on sale of held-to-maturity securities 2us-gaap_HeldToMaturitySecuritiesSoldSecurityRealizedGainLoss 0us-gaap_HeldToMaturitySecuritiesSoldSecurityRealizedGainLoss  
Carrying value of securities reclassified to available-for-sale 94c_AvailableForSaleSecuritiesTransferredFromHeldToMaturityDuringThePeriodCarryingValue 52c_AvailableForSaleSecuritiesTransferredFromHeldToMaturityDuringThePeriodCarryingValue  
OTTI losses on securities reclassified to available-for-sale (5)c_AvailableForSaleSecuritiesTransferredFromHeldToMaturityDuringThePeriodOtherThanTemporaryImpairmentLosses (8)c_AvailableForSaleSecuritiesTransferredFromHeldToMaturityDuringThePeriodOtherThanTemporaryImpairmentLosses  
Securities available-for-sale      
Amortized cost 293,614us-gaap_AvailableForSaleSecuritiesAmortizedCost   299,381us-gaap_AvailableForSaleSecuritiesAmortizedCost
Gross unrealized gains 3,141us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax   2,527us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Gross unrealized losses 1,516us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax   1,765us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Fair value 295,239us-gaap_AvailableForSaleSecurities   300,143us-gaap_AvailableForSaleSecurities
Unrealized gains from impact of minority investments and the related allocations and pick up of unrealized gains and losses of AFS securities 22c_AvailableforSaleSecuritiesGrossUnrealizedGainImpactofMinorityInvestmentsandRelatedAllocationsandPickupofUnrealizedGains   27c_AvailableforSaleSecuritiesGrossUnrealizedGainImpactofMinorityInvestmentsandRelatedAllocationsandPickupofUnrealizedGains
Unrealized gains from impact of securities reclassified to held-for-sale 86c_AvailableforSaleSecuritiesGrossUnrealizedGainImpactofSecuritiesReclassifiedtoHeldforSale    
Unrealized loss from impact of securities reclassified to held-for-sale 5c_AvailableforSaleSecuritiesGrossUnrealizedLossImpactofSecuritiesReclassifiedtoHeldforSale    
Mortgage-backed securities - U.S. agency-sponsored      
Securities available-for-sale      
Amortized cost 38,433us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesIssuedByUSGovernmentCorporationsAndAgenciesMember		   35,647us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesIssuedByUSGovernmentCorporationsAndAgenciesMember
Gross unrealized gains 744us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesIssuedByUSGovernmentCorporationsAndAgenciesMember		   603us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesIssuedByUSGovernmentCorporationsAndAgenciesMember
Gross unrealized losses 93us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesIssuedByUSGovernmentCorporationsAndAgenciesMember		   159us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesIssuedByUSGovernmentCorporationsAndAgenciesMember
Fair value 39,084us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesIssuedByUSGovernmentCorporationsAndAgenciesMember		   36,091us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesIssuedByUSGovernmentCorporationsAndAgenciesMember
Mortgage-backed securities - Prime      
Securities available-for-sale      
Amortized cost 7us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesPrimeMember		   12us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesPrimeMember
Gross unrealized gains 2us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesPrimeMember		   0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesPrimeMember
Gross unrealized losses 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesPrimeMember		   0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesPrimeMember
Fair value 9us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesPrimeMember		   12us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesPrimeMember
Mortgage-backed securities - Alt-A      
Securities available-for-sale      
Amortized cost 2us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesAltAMember		   43us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesAltAMember
Gross unrealized gains 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesAltAMember		   1us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesAltAMember
Gross unrealized losses 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesAltAMember		   0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesAltAMember
Fair value 2us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesAltAMember		   44us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= c_MortgageBackedSecuritiesAltAMember
Mortgage-backed securities - Non-U.S. residential      
Securities available-for-sale      
Amortized cost 7,209us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= c_ForeignResidentialMortgageBackedSecuritiesMember		   8,247us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= c_ForeignResidentialMortgageBackedSecuritiesMember
Gross unrealized gains 61us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= c_ForeignResidentialMortgageBackedSecuritiesMember		   67us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= c_ForeignResidentialMortgageBackedSecuritiesMember
Gross unrealized losses 5us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= c_ForeignResidentialMortgageBackedSecuritiesMember		   7us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= c_ForeignResidentialMortgageBackedSecuritiesMember
Fair value 7,265us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= c_ForeignResidentialMortgageBackedSecuritiesMember		   8,307us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= c_ForeignResidentialMortgageBackedSecuritiesMember
Mortgage-backed securities - Commercial      
Securities available-for-sale      
Amortized cost 500us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		   551us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Gross unrealized gains 8us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		   6us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Gross unrealized losses 1us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		   3us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Fair value 507us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		   554us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Mortgage-backed securities      
Securities available-for-sale      
Amortized cost 46,151us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember		   44,500us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember
Gross unrealized gains 815us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember		   677us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember
Gross unrealized losses 99us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember		   169us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember
Fair value 46,867us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember		   45,008us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_MortgageBackedSecuritiesMember
U.S. Treasury      
Securities available-for-sale      
Amortized cost 110,963us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_USTreasurySecuritiesMember		   110,492us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_USTreasurySecuritiesMember
Gross unrealized gains 1,074us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_USTreasurySecuritiesMember		   353us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_USTreasurySecuritiesMember
Gross unrealized losses 69us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_USTreasurySecuritiesMember		   127us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_USTreasurySecuritiesMember
Fair value 111,968us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_USTreasurySecuritiesMember		   110,718us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_USTreasurySecuritiesMember
Agency obligations      
Securities available-for-sale      
Amortized cost 9,291us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		   12,925us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Gross unrealized gains 102us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		   60us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Gross unrealized losses 2us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		   13us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Fair value 9,391us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		   12,972us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
U.S. Treasury and federal agency securities      
Securities available-for-sale      
Amortized cost 120,254us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember		   123,417us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember
Gross unrealized gains 1,176us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember		   413us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember
Gross unrealized losses 71us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember		   140us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember
Fair value 121,359us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember		   123,690us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember
State and municipal securities      
Securities available-for-sale      
Amortized cost 13,025us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		   13,526us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Gross unrealized gains 137us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		   150us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Gross unrealized losses 917us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		   977us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Fair value 12,245us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		   12,699us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Foreign government      
Securities available-for-sale      
Amortized cost 86,654us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		   90,249us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Gross unrealized gains 688us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		   734us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Gross unrealized losses 283us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		   286us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Fair value 87,059us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		   90,697us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Corporate      
Securities available-for-sale      
Amortized cost 14,363us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember		   12,033us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
Gross unrealized gains 245us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember		   215us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
Gross unrealized losses 54us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember		   91us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
Fair value 14,554us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember		   12,157us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
Asset-backed securities      
Securities available-for-sale      
Amortized cost 11,514us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_AssetBackedSecuritiesMember		   12,534us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_AssetBackedSecuritiesMember
Gross unrealized gains 39us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_AssetBackedSecuritiesMember		   30us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_AssetBackedSecuritiesMember
Gross unrealized losses 49us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_AssetBackedSecuritiesMember		   58us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_AssetBackedSecuritiesMember
Fair value 11,504us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_AssetBackedSecuritiesMember		   12,506us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_AssetBackedSecuritiesMember
Other debt securities      
Securities available-for-sale      
Amortized cost 661us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_OtherDebtSecuritiesMember		   661us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_OtherDebtSecuritiesMember
Gross unrealized gains 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_OtherDebtSecuritiesMember		   0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_OtherDebtSecuritiesMember
Gross unrealized losses 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_OtherDebtSecuritiesMember		   0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_OtherDebtSecuritiesMember
Fair value 661us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_OtherDebtSecuritiesMember		   661us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_OtherDebtSecuritiesMember
Debt securities      
Securities available-for-sale      
Amortized cost 292,622us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_DebtSecuritiesMember		   296,920us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_DebtSecuritiesMember
Gross unrealized gains 3,100us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_DebtSecuritiesMember		   2,219us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_DebtSecuritiesMember
Gross unrealized losses 1,473us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_DebtSecuritiesMember		   1,721us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_DebtSecuritiesMember
Fair value 294,249us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_DebtSecuritiesMember		   297,418us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_DebtSecuritiesMember
Equity securities      
Securities available-for-sale      
Amortized cost 992us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_EquitySecuritiesMember		   2,461us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_EquitySecuritiesMember
Gross unrealized gains 41us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_EquitySecuritiesMember		   308us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_EquitySecuritiesMember
Gross unrealized losses 43us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_EquitySecuritiesMember		   44us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_EquitySecuritiesMember
Fair value 990us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_EquitySecuritiesMember		   2,725us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_EquitySecuritiesMember
Securities available-for-sale (AFS)      
Investment Holdings      
Investments 295,239c_InvestmentsExcludingTradingSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember		   300,143c_InvestmentsExcludingTradingSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_AvailableforsaleSecuritiesMember
Held-to-maturity Securities      
Investment Holdings      
Investments 23,254c_InvestmentsExcludingTradingSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_HeldtomaturitySecuritiesMember		   23,921c_InvestmentsExcludingTradingSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_HeldtomaturitySecuritiesMember
Non-marketable equity securities | Fair value      
Investment Holdings      
Investments 2,570c_InvestmentsExcludingTradingSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ us-gaap_InvestmentTypeAxis
= c_NonMarketableEquitySecuritiesMember		   2,758c_InvestmentsExcludingTradingSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ us-gaap_InvestmentTypeAxis
= c_NonMarketableEquitySecuritiesMember
Non-marketable equity securities | Carried at cost      
Investment Holdings      
Investments $ 5,752c_InvestmentsExcludingTradingSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_PortionAtOtherThanFairValueFairValueDisclosureMember
/ us-gaap_InvestmentTypeAxis
= c_NonMarketableEquitySecuritiesMember		   $ 6,621c_InvestmentsExcludingTradingSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_PortionAtOtherThanFairValueFairValueDisclosureMember
/ us-gaap_InvestmentTypeAxis
= c_NonMarketableEquitySecuritiesMember
[1] Certain prior-period revenue and expense lines and totals were reclassified to conform to the current period’s presentation. See Note 3 to the Consolidated Financial Statements.