SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Mortgage Servicing Rights (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Mortgage servicing rights
Classification of Securitizations
Fair value of capitalized mortgage servicing rights	$ 584	$ 558
Principal amount of loans and other financial instruments	62,000	66,000
Capitalized MSRs
Balance at beginning of period	558	1,564
Originations	58	96
Changes in fair value of MSRs due to changes in inputs and assumptions	54	65
Other changes	(68)	(110)
Sale of MSRs	(18)	(1,057)
Balance at end of period	584	558	$ 1,564
MSR fees
Servicing fees	172	276	484
Late fees	4	10	14
Ancillary fees	8	13	17
Total MSR fees	184	299	$ 515
Mortgage securitizations - Non-agency-sponsored
Re-securitizations
Fair value of re-securitizations deals in which the entity holds a retained interest	16	79
Original par value of re-securitizations deals in which the entity holds a retained interest	271	887
U.S. government-sponsored agency guaranteed
Re-securitizations
Fair value of re-securitizations deals in which the entity holds a retained interest	2,500	2,100
Securities transferred to re-securitization entities	26,300	26,600
Market value of retained interest related to re-securitization transaction	1,400	854
Original fair value of re-securitizations deals in which the entity holds a retained interest	$ 70,900	$ 68,300

No definition available.

Name:	c_ClassificationOfSecuritizationsLineItems
Namespace Prefix:	c_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the fair value of capitalized mortgage servicing rights.

+ References

No definition available.

+ Details

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

- Definition

Represents the fair value of re-securitizations deals in which entity holds retained interest.

+ References

No definition available.

+ Details

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

- Definition

This element represents the amount of market value of interest owned by the entity in private-label re-securitization entities, for current period.

+ References

No definition available.

+ Details

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

- Definition

Represents the amount of original fair value of re-securitizations deals in which the entity holds a retained interest.

+ References

No definition available.

+ Details

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

- Definition

Represents the amount of original par value of re-securitizations deals in which the entity holds a retained interest.

+ References

No definition available.

+ Details

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

- Definition

Re-securitizations [Abstract]

+ References

No definition available.

+ Details

Name:	c_ResecuritizationsAbstract
Namespace Prefix:	c_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the amount of securities transferred to re-securitization entities.

+ References

No definition available.

+ Details

Name:	c_SecuritiesTransferredToReSecuritizationEntities
Namespace Prefix:	c_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

All amounts that are due to the servicer of a financial asset, excluding contractually specified servicing fees and late fees.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122596-111746

+ Details

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

- Definition

All amounts that, per contract, are due to the servicer in exchange for servicing the financial asset and would no longer be received by a servicer if the beneficial owners of the serviced assets (or their trustees or agents) were to exercise their actual or potential authority under the contract to shift the servicing to another servicer. Depending on the servicing contract, those fees may include some or all of the difference between the interest rate collected on the asset being serviced and the rate to be paid to the beneficial owners of the asset.

+ References

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Income from mortgage banking activities (fees and commissions) and servicing real estate mortgages, credit cards, and other financial assets held by others net of any related impairment of capitalized service costs. Also includes any premiums received in lieu of regular servicing fees on such loans only as earned over the life of the loans.

+ References

No definition available.

+ Details

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

- Definition

All amounts that are due to the servicer of a financial asset as a result of the debt holder not meeting the monthly payment terms.

+ References

+ Details

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

- Definition

Amount of transferred financial assets in which the transferor has continuing involvement with the assets underlying the transaction that continue to be recognized in the statement of financial position. Includes, but is not limited to, servicing, recourse, and restrictions on transferor's interests in transferred financial assets.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=109247956&loc=d3e107314-111719

+ Details

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

- Definition

The value of new servicing assets, subsequently measured at fair value, acquired or created during the current period through purchases or from transfers of financial assets.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122625-111746

+ Details

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

- Definition

Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122501-111745

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122625-111746

+ Details

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

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount of increase (decrease) in fair value from changes in the inputs, assumptions, or model used to calculate the fair value of the contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
-Subparagraph (a)(4)(i)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122625-111746

+ Details

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

- Definition

Decrease in servicing assets subsequently measured at fair value resulting from conveyance of servicing rights to unrelated parties.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122625-111746

+ Details

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

- Definition

Amount of increase (decrease) in fair value from changes classified as other for a contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
-Subparagraph (a)(5)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122625-111746

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=c_MortgageServiceRightsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=c_MortgageBackedSecuritiesIssuedByUSGovernmentCorporationsAndAgenciesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: