INVESTMENTS - Recognition and Measurement of OTTI (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
OTTI on Investments disclosures
Total OTTI losses recognized during the period	$ 0	$ 0
Less: portion of impairment loss recognized in AOCI (before taxes)	0	0
Net impairment losses recognized in earnings for debt securities that the Company does not intend to sell nor will likely be required to sell	0	0
Impairment losses recognized in earnings for debt securities that the Company intends to sell, would be more-likely-than-not required to sell or will be subject to an issuer call deemed probable of exercise	3	27
Total OTTI losses recognized in earnings	3	27
Debt securities available-for-sale (AFS)
OTTI on Investments disclosures
Total OTTI losses recognized during the period	0	0
Less: portion of impairment loss recognized in AOCI (before taxes)	0	0
Net impairment losses recognized in earnings for debt securities that the Company does not intend to sell nor will likely be required to sell	0	0
Impairment losses recognized in earnings for debt securities that the Company intends to sell, would be more-likely-than-not required to sell or will be subject to an issuer call deemed probable of exercise	3	27
Total OTTI losses recognized in earnings	3	27
Debt securities held-to-maturity securities (HTM)
OTTI on Investments disclosures
Total OTTI losses recognized during the period	0	0
Less: portion of impairment loss recognized in AOCI (before taxes)	0	0
Net impairment losses recognized in earnings for debt securities that the Company does not intend to sell nor will likely be required to sell	0	0
Impairment losses recognized in earnings for debt securities that the Company intends to sell, would be more-likely-than-not required to sell or will be subject to an issuer call deemed probable of exercise	0	0
Total OTTI losses recognized in earnings	$ 0	$ 0

No definition available.

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

- Definition

For investments that the entity does not intend to sell nor will likely be required to sell, the amount by which the fair value of an investment is less than the amortized cost basis or carrying amount at the balance sheet date and the decline in fair value is deemed to be other than temporary, the amount recognized in earnings for securities (after net of the amount recognized in other comprehensive income).

+ References

No definition available.

+ Details

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

- Definition

The aggregate amount (i) of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis and OTTI losses recognized during the period on investments accounted for under the cost method of accounting.; and (ii) of an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting.

+ References

No definition available.

+ Details

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

- Definition

For investments that the entity does not intend to sell nor will likely be required to sell, the amount by which the fair value of an investment is less than the amortized cost basis or carrying amount at the balance sheet date and the decline in fair value is deemed to be other than temporary, prior to considering whether or not such amount is recognized in earnings or other comprehensive income.

+ References

No definition available.

+ Details

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

- Definition

For investments that the entity does intend to sell or will more-likely-than-not be required to sell before recovery, the amount by which the fair value of an investment is less than the amortized cost basis or carrying amount at the balance sheet date and the decline in fair value is deemed to be other than temporary, the amount recognized in earnings for securities.

+ References

No definition available.

+ Details

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

- Definition

Amount before tax of other than temporary impairment (OTTI) loss on investments in available-for-sale and held-to-maturity debt securities, recognized in other comprehensive loss.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 35
-URI http://asc.fasb.org/extlink&oid=115930623&loc=d3e25362-111560

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=114867106&loc=SL7669646-108580

+ Details

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

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_AvailableforsaleSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_HeldtomaturitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: