FAIR VALUE ELECTIONS - Certain Structured and Non-Structured Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Carrying value of structured notes, disaggregated by type of embedded derivative instrument
Carrying value of structured notes	$ 59,000	$ 48,500
Long-term debt
Carrying value of structured notes, disaggregated by type of embedded derivative instrument
Aggregate unpaid principal balance in excess of (less than) fair value	(3,433)	(5,130)
Short-term borrowings
Carrying value of structured notes, disaggregated by type of embedded derivative instrument
Aggregate unpaid principal balance in excess of (less than) fair value	0	68
Carrying amount \| Long-term debt
Carrying value of structured notes, disaggregated by type of embedded derivative instrument
Carrying amount reported on the consolidated balance sheet	68,071	67,063
Carrying amount \| Short-term borrowings
Carrying value of structured notes, disaggregated by type of embedded derivative instrument
Carrying amount reported on the consolidated balance sheet	7,406	4,683
Interest rate linked
Carrying value of structured notes, disaggregated by type of embedded derivative instrument
Carrying value of structured notes	24,700	16,000
Foreign exchange linked
Carrying value of structured notes, disaggregated by type of embedded derivative instrument
Carrying value of structured notes	800	1,200
Equity linked
Carrying value of structured notes, disaggregated by type of embedded derivative instrument
Carrying value of structured notes	29,500	27,300
Commodity linked
Carrying value of structured notes, disaggregated by type of embedded derivative instrument
Carrying value of structured notes	1,400	1,400
Credit linked
Carrying value of structured notes, disaggregated by type of embedded derivative instrument
Carrying value of structured notes	$ 2,600	$ 2,600

Represents the carrying value of certain structured liabilities.

No definition available.

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

- Definition

This item represents the difference between the aggregate fair value and the aggregate unpaid principal balance of short-term debt instruments that have contractual principal amounts and for which the fair value option has been elected.

+ References

No definition available.

+ Details

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

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

- Definition

This item represents the difference between the aggregate fair value and the aggregate unpaid principal balance of long-term debt instruments that have contractual principal amounts and for which the fair value option has been elected.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 28
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14064-108612

+ Details

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

- Definition

Fair value of financial and nonfinancial obligations.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=c_InterestRateLinkedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=c_ForeignExchangeLinkedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=c_EquityLinkedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=c_CommodityLinkedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=c_CreditLinkedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: