Represents the amount of setoff to be avoided relating to terminated derivatives contracts.

No definition available.

Name:	c_BankruptcyClaimClearingObligationsAmountToAvoidFromSetoffRelatedToTerminatedDerivativesContracts
Namespace Prefix:	c_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of the acquisition price paid in a leveraged buyout in which the entity served as lender and advisor.

+ References

No definition available.

+ Details

Name:	c_BusinessAcquisitionPriceLBO
Namespace Prefix:	c_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the aggregate original purchase amount of collateralized debt obligations and mortgage backed securities underwritten by the entity, which are covered by tolling agreements with investors threatening litigation.

+ References

No definition available.

+ Details

Name:	c_CollateralizedDebtObligationsAndMortgageBackedSecuritiesCoveredByTollingAgreements
Namespace Prefix:	c_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Litigation Settlement, Rebate to Merchants, Basis Points

+ References

No definition available.

+ Details

Name:	c_LitigationSettlementRebatetoMerchantsBasisPoints
Namespace Prefix:	c_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Loss Contingency, Number of Depository Receipts From Conversion Of Foreign Currency Dividends

+ References

No definition available.

+ Details

Name:	c_LossContingencyNumberofDepositoryReceiptsFromConversionOfForeignCurrencyDividends
Namespace Prefix:	c_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Loss Contingency, Number of Mortgage-Backed Securities Trusts

+ References

No definition available.

+ Details

Name:	c_LossContingencyNumberofMortgageBackedSecuritiesTrusts
Namespace Prefix:	c_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Loss Contingency, Number of Mortgage-Backed Securities Trusts Allowed to Proceed

+ References

No definition available.

+ Details

Name:	c_LossContingencyNumberofMortgageBackedSecuritiesTrustsAllowedtoProceed
Namespace Prefix:	c_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Loss Contingency, Number of Mortgage-Backed Securities Trusts Dismissed from Claims

+ References

No definition available.

+ Details

Name:	c_LossContingencyNumberofMortgageBackedSecuritiesTrustsDismissedfromClaims
Namespace Prefix:	c_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Loss Contingency, Private-label Securitizations, Number

+ References

No definition available.

+ Details

Name:	c_LossContingencyPrivatelabelSecuritizationsNumber
Namespace Prefix:	c_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the collection period of interchange fees.

+ References

No definition available.

+ Details

Name:	c_PeriodOfCollectingInterchangeFees
Namespace Prefix:	c_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount or range of possible amounts of gain that could be realized upon the resolution of a contingency.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 30
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6397426&loc=d3e17499-108355

+ Details

Name:	us-gaap_GainContingencyUnrecordedAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount awarded from other party in judgment or settlement of litigation.

+ References

No definition available.

+ Details

Name:	us-gaap_LitigationSettlementAmountAwardedFromOtherParty
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount awarded to other party in judgment or settlement of litigation.

+ References

No definition available.

+ Details

Name:	us-gaap_LitigationSettlementAmountAwardedToOtherParty
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_LossContingenciesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of claims settled.

+ References

No definition available.

+ Details

Name:	us-gaap_LossContingencyClaimsSettledNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of damages awarded to the plaintiff in the legal matter.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14435-108349

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14557-108349

+ Details

Name:	us-gaap_LossContingencyDamagesAwardedValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The value (monetary amount) of the award the plaintiff seeks in the legal matter.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14557-108349

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14435-108349

+ Details

Name:	us-gaap_LossContingencyDamagesSoughtValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The total number of new claims filed pertaining to a loss contingency during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14435-108349

+ Details

Name:	us-gaap_LossContingencyNewClaimsFiledNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of plaintiffs that have filed claims pertaining to a loss contingency.

+ References

No definition available.

+ Details

Name:	us-gaap_LossContingencyNumberOfPlaintiffs
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The difference between the maximum amount of estimated loss and the amount recorded as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 05
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6395354&loc=d3e12631-108344

+ Details

Name:	us-gaap_LossContingencyRangeOfPossibleLossPortionNotAccrued
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of cash paid for the settlement of litigation or for other legal issues during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3536-108585

+ Details

Name:	us-gaap_PaymentsForLegalSettlements
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=c_MortgageBackedSecuritiesTrusteeActionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=c_ResidentialMortgageBackedSecuritiesInvestorActionsAndRepurchaseClaimsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=c_LehmanBrothersBankruptcyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=c_TribuneCompanyBankruptcyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LitigationCaseAxis=c_CitigroupIncSecuritiesLitigationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LitigationCaseAxis=c_LehmanBrothersHoldingsInc.Vs.CitibankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LitigationCaseAxis=c_AntitrustLitigationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LitigationCaseAxis=c_LiborbasedFinancialInstrumentsAntitrustLitigationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LitigationCaseAxis=c_InterchangeFeesLitigationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LitigationCaseAxis=c_OceanografiaFraudandRelatedMattersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LitigationCaseAxis=c_ParmalatMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=c_MilanCourtofAppealMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=c_CitibankNASubsidiaryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

CONTINGENCIES (Details) $ in Millions, € in Billions											12 Months Ended	144 Months Ended
CONTINGENCIES (Details) $ in Millions, € in Billions	Jan. 10, 2018 USD ($)	Oct. 13, 2017 USD ($)	Oct. 06, 2017 USD ($)	Jul. 27, 2017 USD ($)	Feb. 26, 2016 USD ($) plaintiff	Dec. 15, 2015 USD ($)	Jun. 16, 2015 EUR (€)	Aug. 29, 2014 USD ($)	Jan. 14, 2014 USD ($)	Oct. 20, 2008 USD ($) counterclaim	Dec. 31, 2017 USD ($) action securitization	Dec. 31, 2017 USD ($) Depository_receipts	Sep. 07, 2016 trust	Sep. 08, 2015 trust	Nov. 24, 2014 trust	Feb. 08, 2012 USD ($)
Contingencies
Possible unaccrued loss											$ 1,000.0	$ 1,000.0
Number of depository receipts \| Depository_receipts												35
Mortgage-backed securities trustee actions
Contingencies
Number of MBS trusts \| trust															27
Number of MBS trusts dismissed from claims \| trust														24
Number of MBS trusts allowed to proceed with claim \| trust													3	3
Residential Mortgage-Backed Securities Investor Actions and Repurchase Claims
Contingencies
Number of actions filed \| action											6
Number of private-label securitizations \| securitization											4
Lehman Brothers Bankruptcy
Contingencies
Proofs of claims																$ 2,600.0
Bankruptcy claims clearing obligations amount to avoid from setoff related to terminated derivative contracts																$ 2,000.0
Tribune Company bankruptcy
Contingencies
Business acquisition price LBO											$ 11,000.0	$ 11,000.0
In Re Citigroup Inc. Securities Litigation \| Mortgage-backed securities trustee actions
Contingencies
Aggregate original purchase amount of the purchases covered by tolling agreements											$ 500.0	$ 500.0
Lehman Brothers Bankruptcy Proceedings
Contingencies
Settlement amount awarded from other party		$ 350.0
Antitrust Litigation
Contingencies
Payments for legal settlements						$ 400.0
LIBOR-based Financial Instruments Antitrust Litigation
Contingencies
Settlement amount awarded to other party			$ 130.0	$ 33.4
Interchange Fees Litigation
Contingencies
Settlement amount awarded to other party									$ 6,050.0
Rebate to merchants (basis points)									0.0001
Period of collecting interchange fees									8 months
Oceanografia Fraud and Related Matters
Contingencies
Number of plaintiffs \| plaintiff					39
Loss contingency damages sought					$ 1,100.0
Parmalat
Contingencies
Number of claims settled \| counterclaim										3
Damages awarded								$ 431.0		$ 431.0
Parmalat \| Milan Court of Appeal \| Citibank, N.A.
Contingencies
Loss contingency damages sought \| €							€ 1.8
Subsequent event \| LIBOR-based Financial Instruments Antitrust Litigation
Contingencies
Settlement amount awarded to other party	$ 23.0