LOANS - Corporate Loans Credit Quality Indicators (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Loans receivable
Loans, net of unearned income	$ 622,444	$ 644,635
Loans at fair value	5,513	5,901
Corporate
Loans receivable
Loans, net of unearned income	289,071	274,665
Loans, total non-accrual	1,558	1,183
Loans at fair value	5,476	5,858
Corporate \| Commercial and industrial
Loans receivable
Loans, net of unearned income	118,075	110,389
Loans, total non-accrual	1,042	575
Corporate \| Financial institutions
Loans receivable
Loans, net of unearned income	64,832	67,832
Loans, total non-accrual	165	250
Corporate \| Mortgage and real estate
Loans receivable
Loans, net of unearned income	43,547	38,473
Loans, total non-accrual	236	252
Corporate \| Lease financing
Loans receivable
Loans, net of unearned income	1,976	2,113
Loans, total non-accrual	75	51
Corporate \| Other
Loans receivable
Loans, net of unearned income	55,156	49,949
Loans, total non-accrual	40	55
Corporate \| Private Banking loans managed on a delinquency basis
Loans receivable
Loans, net of unearned income	20,044	18,616
Corporate \| Investment Grade
Loans receivable
Loans, net of unearned income	210,697	200,987
Corporate \| Investment Grade \| Commercial and industrial
Loans receivable
Loans, net of unearned income	84,088	80,812
Corporate \| Investment Grade \| Financial institutions
Loans receivable
Loans, net of unearned income	55,722	56,154
Corporate \| Investment Grade \| Mortgage and real estate
Loans receivable
Loans, net of unearned income	19,735	16,068
Corporate \| Investment Grade \| Lease financing
Loans receivable
Loans, net of unearned income	1,627	1,669
Corporate \| Investment Grade \| Other
Loans receivable
Loans, net of unearned income	49,525	46,284
Corporate \| Non-Investment Grade
Loans receivable
Loans, net of unearned income	52,854	49,204
Corporate \| Non-Investment Grade \| Commercial and industrial
Loans receivable
Loans, net of unearned income	32,946	29,003
Loans, total non-accrual	1,042	575
Corporate \| Non-Investment Grade \| Financial institutions
Loans receivable
Loans, net of unearned income	8,945	11,429
Loans, total non-accrual	165	250
Corporate \| Non-Investment Grade \| Mortgage and real estate
Loans receivable
Loans, net of unearned income	3,540	3,587
Loans, total non-accrual	236	252
Corporate \| Non-Investment Grade \| Lease financing
Loans receivable
Loans, net of unearned income	274	393
Loans, total non-accrual	75	51
Corporate \| Non-Investment Grade \| Other
Loans receivable
Loans, net of unearned income	5,591	3,609
Loans, total non-accrual	$ 40	$ 55

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.

No definition available.

+ Details

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

- Definition

Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 7

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5111-111524

+ Details

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

- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5074-111524

+ Details

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

- Definition

Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Glossary Loan

-URI http://asc.fasb.org/extlink&oid=6517073

+ Details

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

- 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_LeaseFinancingMember
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_PrivateBankingLoansManagedOnDelinquencyBasisMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ExternalCreditRatingByGroupingAxis=us-gaap_ExternalCreditRatingInvestmentGradeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ExternalCreditRatingByGroupingAxis=us-gaap_ExternalCreditRatingNonInvestmentGradeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: