SECURITIZATIONS AND VARIABLE INTEREST ENTITIES - Municipal Securities Tender Option Bond Trusts (Details) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($) trust	Dec. 31, 2016 USD ($)
Variable Interest Entity
Number of TOB trusts \| trust	2
Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
Municipal bonds owned by trusts, that have credit guarantee provided by the Company	$ 62	$ 82
Liquidity agreements, customer TOB trust	3,200	2,900
Notional amount of offsetting reimbursement agreements	2,000	2,100
Liquidity agreements, other trusts	$ 6,100	7,400
Maximum \| Municipal securities tender option bond trusts (TOBs)
Variable Interest Entity
The threshold ownership percentage on Residual value of customers TOBs for which the reimbursement agreement applied	25.00%
Client intermediation
Variable Interest Entity
Proceeds from new securitizations	$ 1,100	$ 2,300

This element represents the amount of liquidity agreements for customer TOB trusts.

No definition available.

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

- Definition

Represents the amount of liquidity arrangements for other non-consolidated proprietary trusts.

+ References

No definition available.

+ Details

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

- Definition

This element represents the amount of municipal bonds owned by trusts that have a credit guarantee provided by the entity.

+ References

No definition available.

+ Details

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

- Definition

The notional amount of reimbursement agreements in place, which offset liquidity agreements for customer TOB trusts.

+ References

No definition available.

+ Details

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

- Definition

Number of TOB Trusts

+ References

No definition available.

+ Details

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

- Definition

Represent the threshold of ownership percentage on Residual value of customers TOBs in which if the customers own less than the threshold will have to reimburse the entity for any payment made under the liquidity arrangement.

+ References

No definition available.

+ Details

Name:	c_TheThresholdOwnershipPercentageOnResidualValueByCustomerTOBInWhichReimbursementAgreementApplied
Namespace Prefix:	c_
Data Type:	num:percentItemType
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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=82924980&loc=d3e107207-111719

+ Details

Name:	us-gaap_CashFlowsBetweenTransfereeAndTransferorProceedsFromNewTransfers
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=c_MunicipalSecuritiesTenderOptionBondTrustsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=c_ClientIntermediationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: