FAIR VALUE MEASUREMENT - Valuation Techniques and Inputs for Level 3 Nonrecurring Fair Value Measurements (Details)	12 Months Ended
Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Nonrecurring fair value changes included in earnings
Nonrecurring fair value measurements included in earnings	$ 157,000,000	$ (117,000,000)
Loans held-for-sale
Nonrecurring fair value changes included in earnings
Nonrecurring fair value measurements included in earnings	(13,000,000)	(26,000,000)
Other real estate owned
Nonrecurring fair value changes included in earnings
Nonrecurring fair value measurements included in earnings	(2,000,000)	(4,000,000)
Loans
Nonrecurring fair value changes included in earnings
Nonrecurring fair value measurements included in earnings	(22,000,000)	(87,000,000)
Non-marketable equity securities
Nonrecurring fair value changes included in earnings
Nonrecurring fair value measurements included in earnings	194,000,000	0
Nonrecurring \| Level 3
Valuation techniques and inputs
Loans held-for-sale	1,794,000,000	3,609,000,000
Nonrecurring \| Level 3 \| Price-based
Valuation techniques and inputs
Loans held-for-sale	1,729,000,000	3,186,000,000
Other Real Estate	15,000,000	42,000,000
Loans, Fair Value Disclosure		133,000,000
Nonrecurring \| Level 3 \| Discounted Cash Flow
Valuation techniques and inputs
Loans, Fair Value Disclosure		129,000,000
Nonrecurring \| Level 3 \| Recovery Analysis
Valuation techniques and inputs
Other Real Estate	2,000,000
Loans, Fair Value Disclosure	251,000,000	127,000,000
Nonrecurring \| Level 3 \| Recovery Analysis \| Minimum
Nonrecurring fair value changes included in earnings
Loans, Measurement Input, Value	2.60
Nonrecurring \| Level 3 \| Recovery Analysis \| Maximum
Nonrecurring fair value changes included in earnings
Loans, Measurement Input, Value	85.04
Nonrecurring \| Level 3 \| Recovery Analysis \| Weighted Average
Nonrecurring fair value changes included in earnings
Loans, Measurement Input, Value	28.21
Nonrecurring \| Level 3 \| Non-marketable equity securities \| Price-based
Nonrecurring fair value changes included in earnings
Alternative investment funds, fair value	66,000,000
Price \| Nonrecurring \| Level 3 \| Price-based \| Minimum
Valuation techniques and inputs
Loans Held-for-sale, Measurement Input, Value	0.79	77.93
Other Real Estate Owned, Measurement Input, Value		30.00
Nonrecurring fair value changes included in earnings
Loans, Measurement Input, Value		2.80
Alternative Investment, Measurement Input	45.80
Price \| Nonrecurring \| Level 3 \| Price-based \| Maximum
Valuation techniques and inputs
Loans Held-for-sale, Measurement Input, Value	100.00	100.00
Other Real Estate Owned, Measurement Input, Value		50.36
Nonrecurring fair value changes included in earnings
Loans, Measurement Input, Value		100.00
Alternative Investment, Measurement Input	1,514.00
Price \| Nonrecurring \| Level 3 \| Price-based \| Weighted Average
Valuation techniques and inputs
Loans Held-for-sale, Measurement Input, Value	69.52	99.26
Other Real Estate Owned, Measurement Input, Value		49.09
Nonrecurring fair value changes included in earnings
Loans, Measurement Input, Value		62.46
Alternative Investment, Measurement Input	570.26
Price \| Nonrecurring \| Level 3 \| Recovery Analysis \| Minimum
Valuation techniques and inputs
Other Real Estate Owned, Measurement Input, Value	56.30
Price \| Nonrecurring \| Level 3 \| Recovery Analysis \| Maximum
Valuation techniques and inputs
Other Real Estate Owned, Measurement Input, Value	83.08
Price \| Nonrecurring \| Level 3 \| Recovery Analysis \| Weighted Average
Valuation techniques and inputs
Other Real Estate Owned, Measurement Input, Value	58.27
Appraised Value \| Nonrecurring \| Level 3 \| Price-based \| Minimum
Valuation techniques and inputs
Other Real Estate Owned, Measurement Input, Value	8,394,102	20,278
Appraised Value \| Nonrecurring \| Level 3 \| Price-based \| Maximum
Valuation techniques and inputs
Other Real Estate Owned, Measurement Input, Value	8,394,102	8,091,760
Appraised Value \| Nonrecurring \| Level 3 \| Price-based \| Weighted Average
Valuation techniques and inputs
Other Real Estate Owned, Measurement Input, Value	$ 8,394,102	4,016,665
Appraised Value \| Nonrecurring \| Level 3 \| Recovery Analysis \| Minimum
Nonrecurring fair value changes included in earnings
Loans, Measurement Input, Value		0
Appraised Value \| Nonrecurring \| Level 3 \| Recovery Analysis \| Maximum
Nonrecurring fair value changes included in earnings
Loans, Measurement Input, Value		45,500,000
Appraised Value \| Nonrecurring \| Level 3 \| Recovery Analysis \| Weighted Average
Nonrecurring fair value changes included in earnings
Loans, Measurement Input, Value		$ 38,785,667
Discount To Price \| Nonrecurring \| Level 3 \| Price-based \| Minimum
Valuation techniques and inputs
Other Real Estate Owned, Measurement Input		0.3400
Discount To Price \| Nonrecurring \| Level 3 \| Price-based \| Maximum
Valuation techniques and inputs
Other Real Estate Owned, Measurement Input		0.3400
Discount To Price \| Nonrecurring \| Level 3 \| Price-based \| Weighted Average
Valuation techniques and inputs
Other Real Estate Owned, Measurement Input		0.3400
Discount To Price \| Nonrecurring \| Level 3 \| Recovery Analysis \| Minimum
Valuation techniques and inputs
Other Real Estate Owned, Measurement Input	0.1300
Discount To Price \| Nonrecurring \| Level 3 \| Recovery Analysis \| Maximum
Valuation techniques and inputs
Other Real Estate Owned, Measurement Input	0.1300
Discount To Price \| Nonrecurring \| Level 3 \| Recovery Analysis \| Weighted Average
Valuation techniques and inputs
Other Real Estate Owned, Measurement Input	0.1300
Recovery rate \| Nonrecurring \| Level 3 \| Discounted Cash Flow \| Minimum
Valuation techniques and inputs
Loans, Measurement Input		0.5000
Recovery rate \| Nonrecurring \| Level 3 \| Discounted Cash Flow \| Maximum
Valuation techniques and inputs
Loans, Measurement Input		1.0000
Recovery rate \| Nonrecurring \| Level 3 \| Discounted Cash Flow \| Weighted Average
Valuation techniques and inputs
Loans, Measurement Input		0.6359
Recovery rate \| Nonrecurring \| Level 3 \| Recovery Analysis \| Minimum
Valuation techniques and inputs
Loans, Measurement Input	0.3060
Recovery rate \| Nonrecurring \| Level 3 \| Recovery Analysis \| Maximum
Valuation techniques and inputs
Loans, Measurement Input	1.0000
Recovery rate \| Nonrecurring \| Level 3 \| Recovery Analysis \| Weighted Average
Valuation techniques and inputs
Loans, Measurement Input	0.5051

Alternative Investment, Measurement Input, Value

No definition available.

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

- Definition

Amount of gain (loss) recognized in the income statement for an asset measured on a nonrecurring basis using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

No definition available.

+ Details

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

- Definition

Loans, Fair Value Disclosure

+ References

No definition available.

+ Details

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

- Definition

Loans Held-for-sale, Measurement Input, Value

+ References

No definition available.

+ Details

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

- Definition

Loans, Measurement Input

+ References

No definition available.

+ Details

Name:	c_LoansMeasurementInput
Namespace Prefix:	c_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Loans, Measurement Input, Value

+ References

No definition available.

+ Details

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

- Definition

Other Real Estate Owned, Measurement Input, Value

+ References

No definition available.

+ Details

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

- Definition

Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=SL6742756-110258

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 35
-Paragraph 54B
-URI http://asc.fasb.org/extlink&oid=99392525&loc=SL7495116-110257

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.

+ References

No definition available.

+ Details

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

- Definition

Carrying amount of other real estate not separately disclosed, including, but not limited to, real estate investments, real estate loans that qualify as investments in real estate, and premises that are no longer used in operations may also be included in real estate owned. This does not include real estate assets taken in settlement of troubled loans through surrender or foreclosure.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.10(4))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

Value of input used to measure real estate owned by financial institution by financial institution which is not directly related to its business.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_OtherRealEstateOwnedMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_FairValueByAssetClassAxis=c_LoanHeldForSaleMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_RealEstateAcquiredInSatisfactionOfDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=c_LoanHeldForInvestmentImpairedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=c_NonMarketableEquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsNonrecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=us-gaap_MarketApproachValuationTechniqueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=us-gaap_ValuationTechniqueDiscountedCashFlowMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=c_RecoveryAnalysisValuationTechniqueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_WeightedAverageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=c_PriceBasedValuationTechniqueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=c_MeasurementInputPriceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputAppraisedValueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=c_MeasurementInputDiscountToPriceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=c_MeasurementInputRecoveryRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: