GOODWILL AND INTANGIBLE ASSETS - Changes in Intangible Assets (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2017, Intangible assets (excluding MSRs)	$ 4,588
Acquisitions/ divestitures	427
Amortization	(268)
FX translation and other	(18)
Net carrying amount at June 30, 2018, Intangible assets (excluding MSRs)	4,729	$ 4,588
Mortgage servicing rights (MSRs)	596	558
Total intangible assets	5,325	5,146
Indefinite-lived intangible assets
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2017, Intangible assets (excluding MSRs)	244
Acquisitions/ divestitures	0
Amortization	0
FX translation and other	(28)
Net carrying amount at June 30, 2018, Intangible assets (excluding MSRs)	216	244
Purchased credit card relationships
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2017, Intangible assets (excluding MSRs)	1,539
Acquisitions/ divestitures	425
Amortization	(74)
FX translation and other	(2)
Net carrying amount at June 30, 2018, Intangible assets (excluding MSRs)	1,888	1,539
Credit card contract related intangibles
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2017, Intangible assets (excluding MSRs)	2,589
Acquisitions/ divestitures	2
Amortization	(171)
FX translation and other	(1)
Net carrying amount at June 30, 2018, Intangible assets (excluding MSRs)	2,419	2,589
Core deposit intangibles
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2017, Intangible assets (excluding MSRs)	11
Acquisitions/ divestitures	0
Amortization	(4)
FX translation and other	0
Net carrying amount at June 30, 2018, Intangible assets (excluding MSRs)	7	11
Other customer relationships
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2017, Intangible assets (excluding MSRs)	187
Acquisitions/ divestitures	0
Amortization	(12)
FX translation and other	10
Net carrying amount at June 30, 2018, Intangible assets (excluding MSRs)	185	187
Present value of future profits
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2017, Intangible assets (excluding MSRs)	4
Acquisitions/ divestitures	0
Amortization	0
FX translation and other	0
Net carrying amount at June 30, 2018, Intangible assets (excluding MSRs)	4	4
Other
Finite and Indefinite-lived Intangible Assets
Net carrying amount at December 31, 2017, Intangible assets (excluding MSRs)	14
Acquisitions/ divestitures	0
Amortization	(7)
FX translation and other	3
Net carrying amount at June 30, 2018, Intangible assets (excluding MSRs)	$ 10	$ 14
American Airlines, Sears, The Home Depot, Costco and AT&T \| Customer Concentration Risk \| Intangible Assets, Excluding Mortgage Servicing Rights \| Credit card contract related intangibles
Finite and Indefinite-lived Intangible Assets
Concentration risk, percentage	97.00%	97.00%

No definition available.

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

- Definition

Sum of the carrying amounts of all intangible assets, excluding goodwill and excluding mortgage servicing rights, as of the balance sheet date, net of accumulated amortization and impairment charges.

+ References

No definition available.

+ Details

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

- Definition

Intangible Assets, Excluding Mortgage Servicing Rights Acquisitions (Divestitures)

+ References

No definition available.

+ Details

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

- Definition

This element represents the aggregate amount of amortization of intangible assets excluding mortgage servicing rights (MSRs) during the period.

+ References

No definition available.

+ Details

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

- Definition

This element represents net changes to the recorded value of intangible assets excluding mortgage servicing rights (MSRs), due to foreign currency translation adjustments and purchase accounting adjustments.

+ References

No definition available.

+ Details

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

- Definition

This element represents the sum of the carrying amounts of all intangible assets excluding goodwill but including mortgage servicing rights, as of the balance sheet date that is net of accumulated amortization and impairment charges.

+ References

No definition available.

+ Details

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

- Definition

The amount of mortgage servicing rights (MSRs) as of the balance sheet date.

+ References

No definition available.

+ Details

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

- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13531-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13537-108611

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

+ Details

Name:	us-gaap_ConcentrationRiskPercentage1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_IndefinitelivedIntangibleAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=c_PurchasedCreditCardRelationshipsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_ContractBasedIntangibleAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_CoreDepositsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=c_OtherCustomerRelationshipsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=c_PresentValueOfFutureProfitsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=c_OtherFiniteLivedIntangibleAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MajorCustomersAxis=c_AmericanAirlinesSearsTheHomeDepotCostcoandATTMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByTypeAxis=us-gaap_CustomerConcentrationRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByBenchmarkAxis=c_IntangibleAssetsExcludingMortgageServicingRightsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: