LOANS - Corporate Troubled Debt Restructurings (Details) - Corporate - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Financing receivable impaired
Carrying value of TDRs modified during the period	$ 98	$ 97
Period within which default occurred post-modification	1 year
Number of days past due, default status	60 days
TDR balance	$ 777	765
TDR loans in payment default	0	0
Commercial and industrial
Financing receivable impaired
Carrying value of TDRs modified during the period	94	93
TDR balance	685	636
TDR loans in payment default	0	0
Financial institutions
Financing receivable impaired
TDR balance	0	13
TDR loans in payment default	0	0
Mortgage and real estate
Financing receivable impaired
Carrying value of TDRs modified during the period	4	4
TDR balance	77	112
TDR loans in payment default	0	0
Other
Financing receivable impaired
TDR balance	15	4
TDR loans in payment default	$ 0	0
Commercial banking loans
Financing receivable impaired
Number of days past due, default status	90 days
TDRs involving changes in the amount and/or timing of principal payments
Financing receivable impaired
Carrying value of TDRs modified during the period	$ 0	0
TDRs involving changes in the amount and/or timing of principal payments \| Commercial and industrial
Financing receivable impaired
Carrying value of TDRs modified during the period	0	0
TDRs involving changes in the amount and/or timing of principal payments \| Mortgage and real estate
Financing receivable impaired
Carrying value of TDRs modified during the period	0	0
TDRs involving changes in the amount and/or timing of interest payments
Financing receivable impaired
Carrying value of TDRs modified during the period	0	0
TDRs involving changes in the amount and/or timing of interest payments \| Commercial and industrial
Financing receivable impaired
Carrying value of TDRs modified during the period	0	0
TDRs involving changes in the amount and/or timing of interest payments \| Mortgage and real estate
Financing receivable impaired
Carrying value of TDRs modified during the period	0	0
TDRs involving changes in the amount and/or timing of both principal and interest payments
Financing receivable impaired
Carrying value of TDRs modified during the period	98	97
TDRs involving changes in the amount and/or timing of both principal and interest payments \| Commercial and industrial
Financing receivable impaired
Carrying value of TDRs modified during the period	94	93
TDRs involving changes in the amount and/or timing of both principal and interest payments \| Mortgage and real estate
Financing receivable impaired
Carrying value of TDRs modified during the period	$ 4	$ 4

This represents the period within which the payment default occurred after the loan modification.

No definition available.

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

- Definition

Represents the classified default number of days for which the loan is past due.

+ References

No definition available.

+ Details

Name:	c_NumberOfDaysPastDueForDefaultStatus
Namespace Prefix:	c_
Data Type:	xbrli:durationItemType
Balance Type:	na
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_FinancingReceivableModificationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of financing receivable modified as troubled debt restructuring.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953676-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=118939409&loc=d3e18845-111554

+ Details

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

- Definition

Amount of financing receivable modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=c_CommercialAndIndustrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=c_LoansToFinancialInstitutionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_MortgagesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=c_OthersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=c_CommercialMarketLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=c_ChangesInAmountOrTimingOfPrincipalPaymentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=c_ChangesInAmountOrTimingOfInterestPaymentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=c_ChangesInAmountOrTimingOfPrincipalAndInterestPaymentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: