Description of Business, Basis of Presentation, and Changes in Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Accounting, Policy [Policy Text Block]	Our accounting and reporting policies conform to accounting principles generally accepted in the United States of America (GAAP). Additionally, where applicable, the policies conform to the accounting and reporting guidelines prescribed by bank regulatory authorities.
Use of Estimates, Policy [Policy Text Block]	The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and that affect income and expenses during the reporting period and related disclosures. In developing the estimates and assumptions, management uses all available evidence; however, actual results could differ because of uncertainties associated with estimating the amounts, timing, and likelihood of possible outcomes. Our most significant estimates pertain to the allowance for loan losses, valuations of automotive lease assets and residuals, fair value of financial instruments, legal and regulatory reserves, and the determination of the provision for income taxes.
Income Tax, Policy [Policy Text Block]	Income Taxes In calculating the provision for interim income taxes, in accordance with Accounting Standards Codification (ASC) 740, Income Taxes, we apply an estimated annual effective tax rate to year-to-date ordinary income. At the end of each interim period, we estimate the effective tax rate expected to be applicable for the full fiscal year. This method differs from that described in Note 1 to the Consolidated Financial Statements in our 2016 Annual Report on Form 10-K, which describes our annual significant income tax accounting policy and related methodology.
Transfers and Servicing of Financial Assets, Transfers of Financial Assets, Policy [Policy Text Block]	Securitizations and Variable Interest Entities We securitize, transfer, and service consumer and commercial automotive loans and operating leases. Securitization transactions typically involve the use of variable interest entities (VIEs) and are accounted for either as sales or secured borrowings. We may retain economic interests in securitized and sold assets, which are generally in the form of senior or subordinated interests, other residual interests, and servicing rights. In order to conclude whether or not a VIE is required to be consolidated, careful consideration and judgment must be given to our continuing involvement with the VIE. In circumstances where we have both the power to direct the activities of the entity that most significantly impact the entity's performance and the obligation to absorb losses or the right to receive benefits of the entity that could be significant, we would conclude that we are the primary beneficiary of the VIE, and would consolidate the entity. Consolidation of the VIE would also preclude us from recording an accounting sale on the transaction. In the case of a consolidated VIE, the accounting is consistent with a secured borrowing (e.g., we continue to carry the loans and we record the related securitized debt on our Condensed Consolidated Balance Sheet). In transactions where we are not determined to be the primary beneficiary of the VIE, we must determine whether or not we achieve a sale for accounting purposes. In order to achieve a sale for accounting purposes, the assets being transferred must be legally isolated, not be constrained by restrictions from further transfer, and be deemed to be beyond our control. If we were to fail any of these three criteria for sale accounting, the transfer would be accounted for as a secured borrowing consistent with the preceding paragraph. Refer to Note 10 to the Condensed Consolidated Financial Statements for discussion on VIEs. Gains or losses on off-balance sheet securitizations take into consideration the fair value of any retained interests, including the value of certain servicing assets or liabilities, if any, which are initially recorded at fair value at the date of sale. The estimate of the fair value of the retained interests and servicing requires us to exercise significant judgment about the timing and amount of future cash flows from the interests. Refer to Note 21 to the Condensed Consolidated Financial Statements for a discussion of fair value estimates. Gains or losses on off-balance sheet securitizations and sales are reported in gain on mortgage and automotive loans, net, in our Condensed Consolidated Statement of Comprehensive Income. Retained interests are classified as securities or as other assets depending on their nature. On December 24, 2016, the risk retention rules under the Dodd-Frank Wall Street Reform and Consumer Protection Act (Dodd-Frank Act) of 2010 became effective, requiring us to retain at least five percent of the credit risk of the assets underlying asset-backed securitizations. This note was updated to address the Dodd-Frank Act risk retention rules and differs from our description in Note 1 to the Consolidated Financial Statements in our 2016 Annual Report on Form 10-K. We retain servicing responsibilities for all of our consumer and commercial automotive loan and operating lease securitizations. We may receive servicing fees for off-balance sheet securitizations based on the securitized loan balances and certain ancillary fees, all of which are reported in servicing fees in the Condensed Consolidated Statement of Comprehensive Income. Typically, the fee we are paid for servicing consumer automotive finance receivables represents adequate compensation, and consequently, does not result in the recognition of a servicing asset or liability. Whether on- or off-balance sheet, the investors in the securitization trusts generally have no recourse to our assets outside of protections afforded through customary market representation and warranty repurchase provisions. Refer to Note 1 to the Consolidated Financial Statements in our 2016 Annual Report on Form 10-K regarding additional significant accounting policies.
New Accounting Pronouncements, Policy [Text Block]	Recently Adopted Accounting Standards Stock Compensation — Improvements to Employee Share-Based Payment Accounting (ASU 2016-09) As of December 31, 2016, we adopted Accounting Standards Update (ASU) 2016-09. The amendments in this update changed several aspects of share-based payment accounting. The amendments allowed for an entity-wide accounting policy election to either account for forfeitures as they occur or estimate the number of awards that are expected to vest. We elected to account for forfeitures as they occur. The amendments modified the tax withholding requirements to allow entities to withhold an amount up to the employee’s maximum individual statutory tax rates without resulting in a liability classification of the award as opposed to limiting the withholding to the minimum statutory tax rates as required under previous accounting guidance. The amendments required that all excess tax benefits and tax deficiencies related to share-based payment awards be recognized in income tax expense or benefit in the income statement in the period in which they occur. The amendments also addressed the classification and presentation of certain items on the cash flow statement. Specifically, cash flows related to excess tax benefits should be classified as an operating activity instead of a financing activity and cash flows related to cash paid to a tax authority by an employer when withholding shares from an employee’s award for tax withholding purposes should be classified as a financing activity. The adoption of these amendments did not have a material impact to the financial statements. Recently Issued Accounting Standards Revenue from Contracts with Customers (ASU 2014-09) and Revenue from Contracts with Customers — Deferral of the Effective Date (ASU 2015-14) In May 2014, the Financial Accounting Standards Board (FASB) issued ASU 2014-09. The purpose of this guidance is to streamline and consolidate existing revenue recognition principles in GAAP and to converge revenue recognition principles with International Financial Reporting Standards (IFRS). The core principle of the amendments is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled to receive in exchange for those goods or services. The amendments include a five step process for consideration of the core principle, guidance on the accounting treatment for costs associated with a contract, and disclosure requirements related to the revenue process. As originally issued, the amendments in ASU 2014-09 were to be effective beginning on January 1, 2017. However, in August 2015, the FASB issued ASU 2015-14, which deferred the effective date of the guidance until January 1, 2018, and permitted early adoption as of the original effective date in ASU 2014-09. The FASB has issued several additional ASUs to clarify guidance and provide implementation support for ASU 2014-09. Management has considered these additional ASUs when assessing the overall impact of ASU 2014-09. The amendments to the revenue recognition principles can be applied upon adoption either through a full retrospective application or on a modified retrospective basis with a cumulative effect adjustment on the date of initial adoption with certain practical expedients. Our implementation efforts to date related to this standard have included identifying revenue streams that are within the scope of this guidance, the evaluation of associated contracts and accounting policies, the evaluation of processes and systems of internal control, and the assessment of disclosure requirements of the standard. A majority of our revenue streams are not within the scope of this ASU as they are governed by other accounting standards. Management has determined that certain revenue streams and contractual arrangements are in scope of this guidance, including deposit fees, revenue on certain noninsurance contracts, brokering commissions through our insurance operations, sales of off-lease vehicles, remarketing fee income through SmartAuction, and commission and fee income generated through Ally Invest. Management does not expect these amendments to impact current revenue recognition patterns for a majority of the in scope revenue streams and contracts. However, we expect that the application of this guidance to noninsurance contracts within our insurance business will result in the deferral of certain amounts we currently recognize as revenue and expense upon the origination of the contract and the immediate recognition of certain expenses upon the origination of the contract that are currently deferred. Additionally, upon implementation we expect to expand our financial statement disclosures as required by the standard. Our assessment is not final; however, we do not expect the impact of the new guidance to these specific contracts to be material to the financial statements. We currently plan to adopt this guidance as of January 1, 2018, and expect to use the modified retrospective approach. Financial Instruments — Recognition and Measurement of Financial Assets (ASU 2016-01) In January 2016, the FASB issued ASU 2016-01. The amendments in this update modify the requirements related to the measurement of certain financial instruments in the statement of financial condition and results of operations. For equity investments (other than investments accounted for using the equity method), entities must measure such instruments at fair value with changes in fair value recognized in net income. Changes in fair value for equity securities will no longer be recognized through other comprehensive income. Reporting entities may continue to elect to measure equity investments that do not have a readily determinable fair value at cost with adjustments for impairment and observable changes in price. In addition, for a liability (other than a derivative liability) that an entity measures at fair value, any change in fair value related to the instrument-specific credit risk, that is the entity’s own-credit, should be presented separately in other comprehensive income and not as a component of net income. The amendments are effective on January 1, 2018, with early adoption permitted solely for the provisions pertaining to instrument-specific credit risk for liabilities measured at fair value. The amendments must be applied on a modified retrospective basis with a cumulative effect adjustment as of the beginning of the fiscal year of initial adoption. While the amendment requiring equity investments to be measured at fair value with changes in fair value recognized in net income will create additional volatility in our Condensed Consolidated Statement of Comprehensive Income, we do not anticipate the other amendments will have a material impact to our financial statements. We currently plan to adopt these amendments on January 1, 2018, and expect to use the modified retrospective approach as required. Leases (ASU 2016-02) In February 2016, the FASB issued ASU 2016-02. The amendments in this update primarily replace the existing accounting requirements for operating leases for lessees. Lessee accounting requirements for finance leases and lessor accounting requirements for operating leases and sales type and direct financing leases (sales type and direct financing leases were both previously referred to as capital leases) are largely unchanged. The amendments require the lessee of an operating lease to record a balance sheet gross-up upon lease commencement by recognizing a right-of-use asset and lease liability equal to the present value of the lease payments. The right-of-use asset and lease liability should be derecognized in a manner that effectively yields a straight line lease expense over the lease term. In addition to the changes to the lessee operating lease accounting requirements, the amendments also change the types of costs that can be capitalized related to a lease agreement for both lessees and lessors for all types of leases. The amendments also require additional disclosures for all lease types for both lessees and lessors. The amendments are effective on January 1, 2019, with early adoption permitted. The amendments must be applied on a modified retrospective basis with a cumulative adjustment to the beginning of the earliest fiscal year presented in the financial statements in the period of adoption. Management is currently evaluating the impact of these amendments. Upon adoption, we expect to record a balance sheet gross-up, reflecting our right-of-use asset and lease liability for our operating leases where we are the lessee (for example, our facility leases). We are currently reviewing our operating lease contracts where we are the lessee to determine the impact of the gross-up and the changes to capitalizable costs. We are also reviewing our leases where we are the lessor to determine the impact of the changes to capitalizable costs. We currently plan to adopt these amendments on January 1, 2019, and expect to use the modified retrospective approach as required. Financial Instruments — Credit Losses (ASU 2016-13) In June 2016, the FASB issued ASU 2016-13. The amendments in this update introduce a new accounting model to measure credit losses for financial assets measured at amortized cost. Credit losses for financial assets measured at amortized cost should be determined based on the total current expected credit losses over the life of the financial asset or group of financial assets. In effect, the financial asset or group of financial assets should be presented at the net amount expected to be collected. Credit losses will no longer be measured as they are incurred for financial assets measured at amortized cost. The amendments also modify the accounting for available-for-sale debt securities whereby credit losses will be recorded through an allowance for credit losses rather than a write-down to the security’s cost basis, which allows for reversals of credit losses when estimated credit losses decline. Credit losses for available-for-sale debt securities should be measured in a manner similar to current GAAP. The amendments are effective on January 1, 2020, with early adoption permitted as of January 1, 2019. The amendments must be applied using a modified retrospective approach with a cumulative-effect adjustment through retained earnings as of the beginning of the fiscal year upon adoption. The new accounting model for credit losses represents a significant departure from existing GAAP, and will likely materially increase the allowance for credit losses with a resulting negative adjustment to retained earnings. Management created a formal working group to govern the implementation of these amendments consisting of key stakeholders from finance, risk, and accounting and is currently evaluating the impact of the amendments. We are in the process of designing and building the models and procedures that will be used to calculate the credit loss reserves in accordance with these amendments. We currently plan to adopt these amendments on January 1, 2020, and expect to use the modified retrospective approach as required. Statement of Cash Flows — Restricted Cash (ASU 2016-18) In November 2016, the FASB issued ASU 2016-18. The amendments in this update require that amounts classified as restricted cash and restricted cash equivalents be included within the beginning-of-period and end-of-period amounts along with cash and cash equivalents on the statement of cash flows. Prior to this ASU, specific guidance on the presentation of changes in restricted cash and restricted cash equivalents within the statement of cash flows did not exist. The amendments are effective on January 1, 2018, with early adoption permitted. The amendments must be applied retrospectively to all periods presented within the statement of cash flows upon adoption. The amendments will not impact financial results, but will result in a change in the presentation of restricted cash and restricted cash equivalents within the statement of cash flows. We currently plan to adopt these amendments on January 1, 2018, and expect to use the retrospective approach as required. Receivables — Nonrefundable Fees and Other Costs: Premium Amortization on Purchased Callable Debt Securities (ASU 2017-08) In March 2017, the FASB issued ASU 2017-08. The amendments in this update require premiums on purchased callable debt securities to be amortized to the security’s earliest call date. Prior to this ASU, premiums and discounts on purchased callable debt securities were generally required to be amortized to the security’s maturity date. The amendments do not require an accounting change for securities held at a discount. The amendments are effective on January 1, 2019, with early adoption permitted. The amendments must be applied using a modified retrospective approach with a cumulative-effect adjustment through retained earnings as of the beginning of the fiscal year upon adoption. Management is currently evaluating the impact of these amendments. We currently plan to adopt these amendments on January 1, 2019, and expect to use the modified retrospective approach as required. Derivatives and Hedging — Targeted Improvements to Accounting for Hedging Activities (ASU 2017-12) In August 2017, the FASB issued ASU 2017-12, which enhances the financial reporting of hedging relationships to better portray the economic results of an entity's risk management activities in its financial statements and make certain targeted improvements to simplify the application of the hedge accounting guidance in current GAAP. The amendments in this update better align an entity's risk management activities and financial reporting for hedging relationships through changes to both the designation and measurement guidance for qualifying hedging relationships and presentation of hedge results. The amendments are effective on January 1, 2019, with early adoption permitted. Entities must apply the amendments to cash flow and net investment hedge relationships that exist on the date of adoption using a modified retrospective approach. All transition requirements and elections must be applied to hedging relationships existing as of the adoption date and the effect of adoption should be reflected as of the beginning of the fiscal year of adoption. The presentation and disclosure requirements must be applied prospectively. We are currently evaluating the impact these amendments will have to our financial statements and are evaluating the potential of early adopting the standard on January 1, 2018.

Disclosure of accounting policy for income taxes, which may include its accounting policies for recognizing and measuring deferred tax assets and liabilities and related valuation allowances, recognizing investment tax credits, operating loss carryforwards, tax credit carryforwards, and other carryforwards, methodologies for determining its effective income tax rate and the characterization of interest and penalties in the financial statements.

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Disclosure of accounting policy for transfers of financial assets, including securitization transactions. The disclosure is generally expected to include how the entity determines whether a transaction is accounted for as a sale or financing, and the initial and subsequent accounting for such a transaction. The disclosure also may address how interests that are obtained and sold, or those that continue to be held, in such transactions are measured and accounted for, and the methodology and significant assumptions used to value such interests, and the gains or losses resulting from the sale.

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