ALLOWANCE FOR CREDIT LOSSES - Allowance for Credit Losses Roll Forward (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Allowance for credit losses
Adjusted ACLL at beginning of year	$ 16,974	$ 16,455	$ 24,956
Gross credit losses on loans	(7,881)	(5,156)	(6,720)
Gross recoveries on loans	1,444	1,367	1,825
Net credit losses on loans (NCLs)	(6,437)	(3,789)	(4,895)
Net reserve builds (releases) for loans	1,272	937	(7,283)
Net specific reserve builds (releases) for loans	77	19	(715)
Total provision for credit losses on loans (PCLL)	7,786	4,745	(3,103)
Other, net	174	(437)	(503)
ACLL at end of year	18,145	16,974	16,455
Allowance for credit losses on unfunded commitments
Allowance for credit losses on unfunded commitments (ACLUC) at beginning of period	2,151	1,871	2,655
Provision (release) for credit losses on unfunded lending commitments	(425)	291	(788)
Other, net	2	(11)	4
ACLUC at end of period	1,728	2,151	1,871
Allowance for credit losses on loans, leases and unfunded lending commitments	19,873	19,125	18,326
Cumulative effect of adoption
Allowance for credit losses
Adjusted ACLL at beginning of year	(352)
ACLL at end of year		(352)
Adjusted balance
Allowance for credit losses
Adjusted ACLL at beginning of year	16,622
ACLL at end of year		16,622
Consumer
Allowance for credit losses
Adjusted ACLL at beginning of year	14,119	14,040	20,180
Gross credit losses on loans	(7,553)	(4,878)	(6,220)
Gross recoveries on loans	1,366	1,267	1,711
Net credit losses on loans (NCLs)	(6,187)	(3,611)	(4,509)
Net reserve builds (releases) for loans	1,440	563	(5,208)
Net specific reserve builds (releases) for loans	38	(46)	(460)
Other, net	186	(438)	(472)
ACLL at end of year	15,431	14,119	14,040
Sales or transfers of various consumer loan portfolios to held-for-sale
Reclasses of consumer ACLL to HFS	0	(350)	(370)
Consumer \| Asia Consumer Loan
Sales or transfers of various consumer loan portfolios to held-for-sale
Reclass consumer to held-for sale	0	(350)	0
Consumer \| Australia Consumer Loan
Sales or transfers of various consumer loan portfolios to held-for-sale
Reclass consumer to held-for sale	0	0	(280)
Consumer \| Philippines Consumer Loan
Sales or transfers of various consumer loan portfolios to held-for-sale
Reclass consumer to held-for sale	0	0	(90)
Consumer \| Cumulative effect of adoption
Allowance for credit losses
Adjusted ACLL at beginning of year	(352)
ACLL at end of year		(352)
Consumer \| Adjusted balance
Allowance for credit losses
Adjusted ACLL at beginning of year	13,767
ACLL at end of year		13,767
Corporate
Allowance for credit losses
Adjusted ACLL at beginning of year	2,855	2,415	4,776
Gross credit losses on loans	(328)	(278)	(500)
Gross recoveries on loans	78	100	114
Net credit losses on loans (NCLs)	(250)	(178)	(386)
Net reserve builds (releases) for loans	(168)	374	(2,075)
Net specific reserve builds (releases) for loans	39	65	(255)
Other, net	(12)	1	(31)
ACLL at end of year	2,714	2,855	2,415
Sales or transfers of various consumer loan portfolios to held-for-sale
FX translation	174	(87)	$ (133)
Corporate \| Cumulative effect of adoption
Allowance for credit losses
Adjusted ACLL at beginning of year	0
ACLL at end of year		0
Corporate \| Adjusted balance
Allowance for credit losses
Adjusted ACLL at beginning of year	$ 2,855
ACLL at end of year		$ 2,855

Financing Receivable, Allowance For Credit Loss, Adjustments, Other

No definition available.

Name:	c_FinancingReceivableAllowanceForCreditLossAdjustmentsOther
Namespace Prefix:	c_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Financing Receivable, Allowance For Credit Loss, Loans Sold

+ References

No definition available.

+ Details

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

- Definition

Financing Receivable, Allowance For Credit Losses, Unfunded Lending Commitments [Roll Forward]

+ References

No definition available.

+ Details

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

- Definition

The reserve to cover probable credit losses related to financing receivable and unfunded lending commitments as of the balance sheet date.

+ References

No definition available.

+ Details

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

- Definition

Provision charged or released during the period against earnings for credit losses on loans and leases receivable that are expected and were collectively evaluated for impairment.

+ References

No definition available.

+ Details

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

- Definition

Provision charged or released during the period against earnings for credit losses on loans and leases receivable that are expected and were individually evaluated for impairment.

+ References

No definition available.

+ Details

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

- Definition

Off-Balance Sheet, Credit Loss, Liability, Other Adjustments, Net

+ References

No definition available.

+ Details

Name:	c_OffBalanceSheetCreditLossLiabilityOtherAdjustmentsNet
Namespace Prefix:	c_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

Amount excluding accrued interest, of allowance for credit loss on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479344/326-20-45-1

+ Details

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

- Definition

Amount of foreign currency translation gain (loss) which decreases (increases) allowance for credit loss on financing receivable. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amount, after recovery, of writeoff of financing receivable, charged against allowance for credit loss.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 79
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479294/326-20-55-79

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (a)
-Subparagraph (4)
-Publisher SEC

+ Details

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

- Definition

Amount of increase in allowance for credit loss on financing receivable from recovery.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 79
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479294/326-20-55-79

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 11B
-Subparagraph (c)(4)
-SubTopic 10
-Topic 310
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-11B

+ Details

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

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

- Definition

Amount of writeoff of financing receivable, charged against allowance for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 79
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479294/326-20-55-79

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-6

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 11B
-Subparagraph (c)(3)
-SubTopic 10
-Topic 310
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-11B

+ Details

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

- Definition

Amount of expense reversal of allowance for credit loss on financing receivable transferred to held-for-sale classification.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481990/310-10-45-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 948
-SubTopic 310
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481852/948-310-45-2

+ Details

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

- Definition

Amount of expected credit loss for credit exposure on off-balance-sheet commitment, including but not limited to, loan commitment, standby letter of credit, financial guarantee not accounted for as insurance. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479344/326-20-45-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 460
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482454/460-10-45-1

+ Details

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

- Definition

Amount of expense (reversal of expense) for expected credit loss for off-balance sheet credit exposure. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging.

+ References

+ Details

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

- Definition

Amount of credit loss expense (reversal of expense) for financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(11))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483589/942-220-S99-1

+ Details

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

- Details

Name:	srt_CumulativeEffectPeriodOfAdoptionAxis=srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CumulativeEffectPeriodOfAdoptionAxis=srt_CumulativeEffectPeriodOfAdoptionAdjustedBalanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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