SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Collateralized Debt and Loan Obligations (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Variable Interest Entity
Term of collateralized loan obligation	12 years
Collateralized loan obligations (CLOs)
Variable Interest Entity
Principal securitized	$ 0	$ 0	$ 133
Proceeds from new securitizations	0	0	133
Cash flows received on retained interests and other net cash flows	72	127	107
Carrying value of retained interests	$ 1,404	$ 3,142	$ 4,079

Finite life of collateralized loan obligations

No definition available.

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

- Definition

Cash Flows between a transferor and a transferee attributable to newly transferred assets related to either a securitization, asset-backed financing arrangement, or similar transfer in which the transferor has continuing involvement with the transferred financial assets underlying the transaction (including, but not limited to, servicing, recourse, and restrictions on transferor's interests in the 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 3
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=118948506&loc=d3e107207-111719

+ Details

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

- Definition

Principal amount of transferred financial assets in which the transferor has continuing involvement with the assets underlying the transaction. Continuing involvement 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)(1)
-URI http://asc.fasb.org/extlink&oid=118948506&loc=d3e107314-111719

+ Details

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

- Definition

The carrying amount (fair value) as of the balance sheet date of the transferor's interests in transferred financial assets (typically securitized receivables, generally not involved in the collection of principal) following transfer of significant portions of such financial instruments to other parties. Such interests could be ancillary cash flows and interest-only strips.

+ 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=118948506&loc=d3e107314-111719

+ Details

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

- Definition

Amount of cash inflow from collection on beneficial interest in securitization of receivables.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3179-108585

+ Details

Name:	us-gaap_ProceedsFromCollectionOfRetainedInterestInSecuritizedReceivables
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_VariableInterestEntityLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

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