BASIS OF PRESENTATION AND ACCOUNTING CHANGES (Policies)	9 Months Ended
Sep. 30, 2017
Accounting Policies [Abstract]
Accounting Changes	ACCOUNTING CHANGES Premium Amortization on Purchased Callable Debt Securities In March 2017, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) No. 2017-08, Receivables—Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities, which amends the amortization period for certain purchased callable debt securities held at a premium. The ASU requires entities to amortize premiums on debt securities by the first call date when the securities have fixed and determinable call dates and prices. The scope of the ASU includes all accounting premiums, such as purchase premiums and cumulative fair value hedge adjustments. The ASU does not change the accounting for discounts, which continue to be recognized over the contractual life of a security. For calendar-year-end entities, the ASU is effective as of January 1, 2019, but it may be early adopted in any interim or year-end period after issuance. Adoption of the ASU is on a modified retrospective basis through a cumulative effect adjustment to retained earnings as of the beginning of the year of adoption. Citi has early adopted the ASU in the second quarter of 2017, with an effective date of January 1, 2017. Adoption of the ASU primarily affected Citi’s available-for-sale (AFS) and held-to-maturity (HTM) portfolios of callable state and municipal securities. The ASU adoption resulted in a net reduction to total stockholders’ equity of $156 million (after tax), effective as of January 1, 2017. This amount is composed of a reduction of approximately $660 million to retained earnings for the incremental amortization of purchase premiums and cumulative hedge adjustments generated under fair value hedges of these callable debt securities. This amount was partially offset by an increase to Accumulated other comprehensive income (loss) (AOCI) of $504 million related to the cumulative fair value hedge adjustments reclassified to retained earnings for AFS securities. Financial statements for periods prior to 2017 were not subject to restatement under the provisions of this ASU. The amortization recorded in the third quarter and for the first nine months of 2017 under the provisions of the ASU is not materially different than the amounts that would have been recorded if the ASU had not been early adopted. Recognition and Measurement of Financial Assets and Financial Liabilities In January 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) No. 2016-01, Financial Instruments—Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities, which addresses certain aspects of recognition, measurement, presentation and disclosure of financial instruments. This ASU requires entities to present separately in AOCI the portion of the total change in the fair value of a liability resulting from a change in the instrument-specific credit risk when the entity has elected to measure the liability at fair value in accordance with the fair value option for financial instruments. It also requires equity investments (except those accounted for under the equity method of accounting or those that result in consolidation of the investee) to be measured at fair value with changes in fair value recognized in net income, thus eliminating eligibility for the current available-for-sale category. However, Federal Reserve Bank and Federal Home Loan Bank stock as well as certain exchange seats will continue to be presented at cost. Citi early adopted only the provisions of this ASU related to presentation of the change in fair value of liabilities for which the fair value option was elected, related to changes in Citigroup’s own credit spreads in AOCI effective January 1, 2016. Accordingly, as of the first quarter of 2016, these amounts are reflected as a component of AOCI, whereas these amounts were previously recognized in Citigroup’s revenues and net income. The impact of adopting this amendment resulted in a cumulative catch-up reclassification from retained earnings to AOCI of an accumulated after-tax loss of approximately $15 million at January 1, 2016. Financial statements for periods prior to 2016 were not subject to restatement under the provisions of this ASU. For additional information, see Note 17, Note 20 and Note 21 to the Consolidated Financial Statements. The Company is evaluating the effects that the other provisions of ASU 2016-01, which are effective January 1, 2018, will have on its Consolidated Financial Statements and related disclosures. FUTURE APPLICATION OF ACCOUNTING STANDARDS Accounting for Financial Instruments—Credit Losses In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326). The ASU introduces a new credit loss model, the Current Expected Credit Losses model (CECL), which requires earlier recognition of credit losses, while also providing additional transparency about credit risk. The CECL model utilizes a lifetime “expected credit loss” measurement objective for the recognition of credit losses for loans, held-to-maturity securities and other receivables at the time the financial asset is originated or acquired. The expected credit losses are adjusted each period for changes in expected lifetime credit losses. For available-for-sale securities where fair value is less than cost, credit-related impairment, if any, will be recognized in an allowance for credit losses and adjusted each period for changes in expected credit risk. This model replaces the multiple existing impairment models in current GAAP, which generally require that a loss be incurred before it is recognized. The CECL model represents a significant change from existing GAAP and may result in material changes to the Company’s accounting for financial instruments. The Company is evaluating the effect that ASU 2016-13 will have on its Consolidated Financial Statements and related disclosures. The impact of the ASU will depend upon the state of the economy and the nature of Citi’s portfolios at the date of adoption. Based on a preliminary analysis performed earlier in 2017 and the environment at that time, the overall impact is estimated to be an approximate 10-20% increase in credit reserves. Moreover, there are still some implementation questions that will need to be resolved that could affect the estimated impact. The ASU will be effective for Citi as of January 1, 2020. Early application is permitted for annual periods beginning January 1, 2019. Revenue Recognition In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers, which requires an entity to recognize the amount of revenue to which it expects to be entitled for the transfer of promised goods or services to customers. The Company will adopt the guidance as of January 1, 2018 using a modified retrospective method with a cumulative-effect adjustment to opening retained earnings. While the guidance will replace most existing revenue recognition guidance in GAAP, the ASU is not applicable to financial instruments and, therefore, will not impact a majority of the Company’s revenues, including net interest income. Based on the Company’s current interpretations of the new guidance, the Company does not expect a material change in the timing or measurement of revenues and the overall impact to net income is expected to be immaterial. The new standard clarified the guidance related to reporting revenue gross as a principal versus net as an agent. The Company has identified transactions, including underwriting activity where Citi is deemed the principal, rather than the agent, which require a gross up of annual revenues and expenses of approximately $0.8 billion. This change in presentation will not have an impact on Income from continuing operations; however, this standard would have impacted Citi’s efficiency ratio by approximately 50 basis points for the nine months ended September 30, 2017. The Company continues to evaluate the effect that the guidance will have on other revenue streams within its scope, including the presentation of certain contract costs, as well as changes in disclosures required by the new guidance. Lease Accounting In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), which is intended to increase transparency and comparability of accounting for lease transactions. The ASU will require lessees to recognize leases on the balance sheet as lease assets and lease liabilities and will require both quantitative and qualitative disclosures regarding key information about leasing arrangements. Lessor accounting is largely unchanged. The guidance is effective beginning January 1, 2019 with an option to early adopt. The Company does not plan to early adopt the ASU. The Company estimates that upon adoption, its Consolidated Balance Sheet will have an approximate $5 billion increase in assets and liabilities. Additionally, the Company estimates an approximate $200 million increase in retained earnings due to the cumulative effect of recognizing previously deferred gains on sale/leaseback transactions. Income Tax Impact of Intra-Entity Transfers of Assets In October 2016, the FASB issued ASU No. 2016-16, Income Taxes—Intra-Entity Transfers of Assets Other Than Inventory, which will require an entity to recognize the income tax consequences of an intra-entity transfer of an asset other than inventory when the transfer occurs. The ASU is effective January 1, 2018. The Company continues to evaluate the impact of this standard, which is expected to increase DTAs, with an associated decrease in prepaid taxes of approximately $500 million. Subsequent Measurement of Goodwill In January 2017, the FASB issued ASU No. 2017-04, Intangibles—Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment. The ASU simplifies the subsequent measurement of goodwill impairment by eliminating the requirement to calculate the implied fair value of goodwill (i.e., the current Step 2 of the goodwill impairment test) to measure a goodwill impairment charge. Under the ASU, the impairment test is simply the comparison of the fair value of a reporting unit with its carrying amount (the current Step 1), with the impairment charge being the deficit in fair value but not exceeding the total amount of goodwill allocated to that reporting unit. The simplified one-step impairment test applies to all reporting units (including those with zero or negative carrying amounts). The ASU is effective for Citi as of January 1, 2020. Early adoption is permitted for interim and annual goodwill impairment testing dates after January 1, 2017. The impact of the ASU will depend upon the performance of the reporting units and the market conditions impacting the fair value of each reporting unit going forward. Clarifying the Definition of a Business In January 2017, the FASB issued ASU No. 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business. The definition of a business directly and indirectly affects many areas of accounting (e.g., acquisitions, disposals, goodwill and consolidation). The ASU narrows the definition of a business by introducing a quantitative screen as the first step, such that if substantially all of the fair value of the gross assets acquired is concentrated in a single identifiable asset or a group of similar identifiable assets, the set of transferred assets and activities is not a business. If the set is not scoped out from the quantitative screen, the entity then evaluates whether the set meets the requirement that a business include, at a minimum, an input and a substantive process that together significantly contribute to the ability to create outputs. The ASU is effective for Citi as of January 1, 2018. The ASU will be applied prospectively, with early adoption permitted. The impact of the ASU will depend upon the acquisition and disposal activities of Citi. If fewer transactions qualify as a business, there could be less initial recognition of goodwill, but also less goodwill allocated to disposals. Changes in Accounting for Pension and Postretirement (Benefit) Expense In March 2017, the FASB issued ASU 2017-07, Compensation—Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost, which changes the income statement presentation of net benefit expense and requires restating the Company’s financial statements for each of the earlier periods presented in Citi’s annual and interim financial statements. The change in presentation is effective for annual and interim periods starting January 1, 2018. The ASU requires that only the service cost component of net benefit expense be included in the Compensation and benefits line on the income statement. The other components of net benefit expense will be required to be presented outside of the Compensation and benefits line and will be presented in Other operating expense. Since both of these income statement line items are part of Operating expenses, total Operating expenses will not change, nor will there be any change in Net income. This change in presentation is not expected to have a material effect on the Compensation and benefits and on Other operating lines in the income statement. The components of the net benefit expense are currently disclosed in Note 7 to the Consolidated Financial Statements. The new standard also changes the components of net benefit expense that are eligible for capitalization when employee costs are capitalized in connection with various activities, such as internally developed software, construction-in-progress, and loan origination costs. Prospectively from January 1, 2018, only the service cost component of net benefit expense may be capitalized. Existing capitalized balances are not affected. The Company is currently evaluating the portion of net benefits cost that continues to be eligible for capitalization and the portion that is not eligible. Hedging In August 2017, the FASB issued ASU No. 2017-12, Targeted Improvements to Accounting for Hedging Activities, which will better align an entity’s risk management activities and financial reporting for hedging relationships through changes to the designation and measurement guidance for qualifying hedging relationships and the presentation of hedge results. The mandatory effective date for calendar year-end public companies is January 1, 2019 but the amendments may be early adopted in any interim or annual period after issuance. The targeted improvements in the ASU will allow Citi increased flexibility to structure hedges of fixed rate instruments and floating rate instruments. Application of the ASU is expected to reduce the amount of ineffectiveness as the revised accounting guidance will better reflect the economics of our risk management activities and will also reduce the volatility associated with foreign currency hedging. The ASU requires the hedging instrument to be presented in the same line item as the hedged item and also requires expanded disclosures. Citi is in the process of evaluating whether to early adopt the standard before the mandatory effective date.

ACCOUNTING CHANGES

Premium Amortization on Purchased Callable Debt Securities

In March 2017, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) No. 2017-08, Receivables—Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities, which amends the amortization period for certain purchased callable debt securities held at a premium. The ASU requires entities to amortize premiums on debt securities by the first call date when the securities have fixed and determinable call dates and prices. The scope of the ASU includes all accounting premiums, such as purchase premiums and cumulative fair value hedge adjustments. The ASU does not change the accounting for discounts, which continue to be recognized over the contractual life of a security.

For calendar-year-end entities, the ASU is effective as of January 1, 2019, but it may be early adopted in any interim or year-end period after issuance. Adoption of the ASU is on a modified retrospective basis through a cumulative effect adjustment to retained earnings as of the beginning of the year of adoption. Citi has early adopted the ASU in the second quarter of 2017, with an effective date of January 1, 2017. Adoption of the ASU primarily affected Citi’s available-for-sale (AFS) and held-to-maturity (HTM) portfolios of callable state and municipal securities. The ASU adoption resulted in a net reduction to total stockholders’ equity of $156 million (after tax), effective as of January 1, 2017. This amount is composed of a reduction of approximately $660 million to retained earnings for the incremental amortization of purchase premiums and cumulative hedge adjustments generated under fair value hedges of these callable debt securities. This amount was partially offset by an increase to Accumulated other comprehensive income (loss) (AOCI) of $504 million related to the cumulative fair value hedge adjustments reclassified to retained earnings for AFS securities.

Financial statements for periods prior to 2017 were not subject to restatement under the provisions of this ASU. The amortization recorded in the third quarter and for the first nine months of 2017 under the provisions of the ASU is not materially different than the amounts that would have been recorded if the ASU had not been early adopted.

Recognition and Measurement of Financial Assets and Financial Liabilities

In January 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) No. 2016-01, Financial Instruments—Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities, which addresses certain aspects of recognition, measurement, presentation and disclosure of financial instruments.

This ASU requires entities to present separately in AOCI the portion of the total change in the fair value of a liability resulting from a change in the instrument-specific credit risk when the entity has elected to measure the liability at fair value in accordance with the fair value option for financial instruments. It also requires equity investments (except those accounted for under the equity method of accounting or those that result in consolidation of the investee) to be measured at fair value with changes in fair value recognized in net income, thus eliminating eligibility for the current available-for-sale category. However, Federal Reserve Bank and Federal Home Loan Bank stock as well as certain exchange seats will continue to be presented at cost.

Citi early adopted only the provisions of this ASU related to presentation of the change in fair value of liabilities for which the fair value option was elected, related to changes in Citigroup’s own credit spreads in AOCI effective January 1, 2016. Accordingly, as of the first quarter of 2016, these amounts are reflected as a component of AOCI, whereas these amounts were previously recognized in Citigroup’s revenues and net income. The impact of adopting this amendment resulted in a cumulative catch-up reclassification from retained earnings to AOCI of an accumulated after-tax loss of approximately $15 million at January 1, 2016. Financial statements for periods prior to 2016 were not subject to restatement under the provisions of this ASU. For additional information, see Note 17, Note 20 and Note 21 to the Consolidated Financial Statements. The Company is evaluating the effects that the other provisions of ASU 2016-01, which are effective January 1, 2018, will have on its Consolidated Financial Statements and related disclosures.

FUTURE APPLICATION OF ACCOUNTING STANDARDS

Accounting for Financial Instruments—Credit Losses

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326). The ASU introduces a new credit loss model, the Current Expected Credit Losses model (CECL), which requires earlier recognition of credit losses, while also providing additional transparency about credit risk.

The CECL model utilizes a lifetime “expected credit loss” measurement objective for the recognition of credit losses for loans, held-to-maturity securities and other receivables at the time the financial asset is originated or acquired. The expected credit losses are adjusted each period for changes in expected lifetime credit losses. For available-for-sale securities where fair value is less than cost, credit-related impairment, if any, will be recognized in an allowance for credit losses and adjusted each period for changes in expected credit risk. This model replaces the multiple existing impairment models in current GAAP, which generally require that a loss be incurred before it is recognized.

The CECL model represents a significant change from existing GAAP and may result in material changes to the Company’s accounting for financial instruments. The Company is evaluating the effect that ASU 2016-13 will have on its Consolidated Financial Statements and related disclosures. The impact of the ASU will depend upon the state of the economy and the nature of Citi’s portfolios at the date of adoption. Based on a preliminary analysis performed earlier in 2017 and the environment at that time, the overall impact is estimated to be an approximate 10-20% increase in credit reserves. Moreover, there are still some implementation questions that will need to be resolved that could affect the estimated impact. The ASU will be effective for Citi as of January 1, 2020. Early application is permitted for annual periods beginning January 1, 2019.

Revenue Recognition

In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers, which requires an entity to recognize the amount of revenue to which it expects to be entitled for the transfer of promised goods or services to customers. The Company will adopt the guidance as of January 1, 2018 using a modified retrospective method with a cumulative-effect adjustment to opening retained earnings. While the guidance will replace most existing revenue recognition guidance in GAAP, the ASU is not applicable to financial instruments and, therefore, will not impact a majority of the Company’s revenues, including net interest income. Based on the Company’s current interpretations of the new guidance, the Company does not expect a material change in the timing or measurement of revenues and the overall impact to net income is expected to be immaterial.

The new standard clarified the guidance related to reporting revenue gross as a principal versus net as an agent. The Company has identified transactions, including underwriting activity where Citi is deemed the principal, rather than the agent, which require a gross up of annual revenues and expenses of approximately $0.8 billion. This change in presentation will not have an impact on Income from continuing operations; however, this standard would have impacted Citi’s efficiency ratio by approximately 50 basis points for the nine months ended September 30, 2017. The Company continues to evaluate the effect that the guidance will have on other revenue streams within its scope, including the presentation of certain contract costs, as well as changes in disclosures required by the new guidance.

Lease Accounting

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), which is intended to increase transparency and comparability of accounting for lease transactions. The ASU will require lessees to recognize leases on the balance sheet as lease assets and lease liabilities and will require both quantitative and qualitative disclosures regarding key information about leasing arrangements. Lessor accounting is largely unchanged. The guidance is effective beginning January 1, 2019 with an option to early adopt. The Company does not plan to early adopt the ASU. The Company estimates that upon adoption, its Consolidated Balance Sheet will have an approximate $5 billion increase in assets and liabilities. Additionally, the Company estimates an approximate $200 million increase in retained earnings due to the cumulative effect of recognizing previously deferred gains on sale/leaseback transactions.

Income Tax Impact of Intra-Entity Transfers of Assets

In October 2016, the FASB issued ASU No. 2016-16, Income Taxes—Intra-Entity Transfers of Assets Other Than Inventory, which will require an entity to recognize the income tax consequences of an intra-entity transfer of an asset other than inventory when the transfer occurs. The ASU is effective January 1, 2018. The Company continues to evaluate the impact of this standard, which is expected to increase DTAs, with an associated decrease in prepaid taxes of approximately $500 million.

Subsequent Measurement of Goodwill

In January 2017, the FASB issued ASU No. 2017-04, Intangibles—Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment. The ASU simplifies the subsequent measurement of goodwill impairment by eliminating the requirement to calculate the implied fair value of goodwill (i.e., the current Step 2 of the goodwill impairment test) to measure a goodwill impairment charge. Under the ASU, the impairment test is simply the comparison of the fair value of a reporting unit with its carrying amount (the current Step 1), with the impairment charge being the deficit in fair value but not exceeding the total amount of goodwill allocated to that reporting unit. The simplified one-step impairment test applies to all reporting units (including those with zero or negative carrying amounts).

The ASU is effective for Citi as of January 1, 2020. Early adoption is permitted for interim and annual goodwill impairment testing dates after January 1, 2017. The impact of the ASU will depend upon the performance of the reporting units and the market conditions impacting the fair value of each reporting unit going forward.

Clarifying the Definition of a Business

In January 2017, the FASB issued ASU No. 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business. The definition of a business directly and indirectly affects many areas of accounting (e.g., acquisitions, disposals, goodwill and consolidation). The ASU narrows the definition of a business by introducing a quantitative screen as the first step, such that if substantially all of the fair value of the gross assets acquired is concentrated in a single identifiable asset or a group of similar identifiable assets, the set of transferred assets and activities is not a business. If the set is not scoped out from the quantitative screen, the entity then evaluates whether the set meets the requirement that a business include, at a minimum, an input and a substantive process that together significantly contribute to the ability to create outputs.

The ASU is effective for Citi as of January 1, 2018. The ASU will be applied prospectively, with early adoption permitted. The impact of the ASU will depend upon the acquisition and disposal activities of Citi. If fewer transactions qualify as a business, there could be less initial recognition of goodwill, but also less goodwill allocated to disposals.

Changes in Accounting for Pension and Postretirement (Benefit) Expense

In March 2017, the FASB issued ASU 2017-07, Compensation—Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost, which changes the income statement presentation of net benefit expense and requires restating the Company’s financial statements for each of the earlier periods presented in Citi’s annual and interim financial statements. The change in presentation is effective for annual and interim periods starting January 1, 2018. The ASU requires that only the service cost component of net benefit expense be included in the Compensation and benefits line on the income statement. The other components of net benefit expense will be required to be presented outside of the Compensation and benefits line and will be presented in Other operating expense. Since both of these income statement line items are part of Operating expenses, total Operating expenses will not change, nor will there be any change in Net income. This change in presentation is not expected to have a material effect on the Compensation and benefits and on Other operating lines in the income statement. The components of the net benefit expense are currently disclosed in Note 7 to the Consolidated Financial Statements.

The new standard also changes the components of net benefit expense that are eligible for capitalization when employee costs are capitalized in connection with various activities, such as internally developed software, construction-in-progress, and loan origination costs. Prospectively from January 1, 2018, only the service cost component of net benefit expense may be capitalized. Existing capitalized balances are not affected. The Company is currently evaluating the portion of net benefits cost that continues to be eligible for capitalization and the portion that is not eligible.

Hedging

In August 2017, the FASB issued ASU No. 2017-12, Targeted Improvements to Accounting for Hedging Activities, which will better align an entity’s risk management activities and financial reporting for hedging relationships through changes to the designation and measurement guidance for qualifying hedging relationships and the presentation of hedge results. The mandatory effective date for calendar year-end public companies is January 1, 2019 but the amendments may be early adopted in any interim or annual period after issuance. The targeted improvements in the ASU will allow Citi increased flexibility to structure hedges of fixed rate instruments and floating rate instruments. Application of the ASU is expected to reduce the amount of ineffectiveness as the revised accounting guidance will better reflect the economics of our risk management activities and will also reduce the volatility associated with foreign currency hedging. The ASU requires the hedging instrument to be presented in the same line item as the hedged item and also requires expanded disclosures. Citi is in the process of evaluating whether to early adopt the standard before the mandatory effective date.