Description of Business, Basis of Presentation, and Changes in Significant Accounting Policies Description of Business, Basis of Presentation, and Changes in Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Accounting, Policy	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. 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, and the determination of the provision for income taxes.
Lessee, Leases, Policy	At contract inception, we determine whether the contract is or contains a lease based on the terms and conditions of the contract. Lease contracts are recognized on our Condensed Consolidated Balance Sheet as right-of-use (ROU) assets and lease liabilities; however, we have elected not to recognize ROU assets and lease liabilities on real estate leases with terms of one year or less. Lease liabilities and their corresponding ROU assets are recorded based on the present value of the future lease payments over the expected lease term. As the interest rate implicit in the lease contract is typically not readily determinable, we utilize our incremental borrowing rate, which is the rate we would incur to borrow on a collateralized basis over a similar term on an amount equal to the lease payments in a similar economic environment. The ROU asset also includes initial direct costs paid less lease incentives received from the lessor. Our lease contracts are generally classified as operating and, as a result, we recognize a single lease cost within other operating expenses on the income statement on a straight-line basis over the lease term. This update to our accounting policy resulted from our adoption of Accounting Standards Update (ASU) 2016-02 on January 1, 2019, as further described within the section below titled Recently Adopted Accounting Standards
Investment, Policy	Premiums on debt securities that have noncontingent call features that are callable at fixed prices on preset dates are amortized to the earliest call date as an adjustment to investment yield. All other premiums and discounts on debt securities are amortized over the stated maturity of the security as an adjustment to investment yield. This method of amortization differs from that described in Note 1 to the Consolidated Financial Statements in our 2018 Annual Report on Form 10-K, which describes our full accounting policy for Investments. This update to our amortization methodology resulted from the adoption of ASU 2017-08 on January 1, 2019, as further described within the section below titled Recently Adopted Accounting Standards.
Income Tax, Policy	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 2018 Annual Report on Form 10-K, which describes our annual significant income tax accounting policy and related methodology.
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncements, Policy	In February 2016, the Financial Accounting Standards Board (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 (previously referred to as capital leases) and lessor accounting requirements for operating leases and sales type and direct financing 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 ROU asset and lease liability equal to the present value of the lease payments. The ROU 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. The amendments also require additional disclosures for all lease types for both lessees and lessors. The FASB issued additional ASUs to clarify the guidance and provide certain practical expedients and an additional transition option. We adopted ASU 2016-02 and the subsequent ASUs that modified ASU 2016-02 (collectively, the amendments) on January 1, 2019. This includes the early adoption of ASU 2019-01, which was issued in March 2019 to amend certain provisions included in ASU 2016-02. We adopted this guidance using the modified retrospective approach on January 1, 2019, and have not adjusted prior period comparative information and will continue to disclose prior period financial information in accordance with the previous lease accounting guidance. We have elected certain practical expedients permitted within the amendments that allow us to not reassess (i) current lease classifications, (ii) whether existing contracts meet the definition of a lease under the amendments to the lease guidance, and (iii) whether current initial direct costs meet the new criteria for capitalization, for all existing leases as of the adoption date. We made an accounting policy election to calculate the impact of adoption using the remaining minimum lease payments and remaining lease term for each contract that was identified as a lease, discounted at our incremental borrowing rate as of the adoption date. The adoption of the amendments resulted in a ROU asset of approximately $161 million from operating leases for our various corporate facilities, a $29 million reduction to accrued expenses and other liabilities for accrued rent and unamortized tenant improvement allowances, and a lease liability of approximately $190 million. The adoption did not change our previously reported Condensed Consolidated Statements of Comprehensive Income and did not result in a cumulative catch-up adjustment to opening retained earnings.
Accounting Standards Update 2017-08
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncements, Policy	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. We adopted the amendments on January 1, 2019, on a modified retrospective basis, which resulted in an increase to our accumulated deficit of $10 million, net of income taxes, partially offset by an $8 million decrease to accumulated other comprehensive loss, net of income taxes.
Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncements, Policy	In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (CECL). 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 recorded under the current incurred loss model 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. We plan to adopt these amendments on January 1, 2020, and expect to utilize the modified retrospective approach as required. The new accounting model for credit losses represents a significant departure from existing GAAP, and will materially increase the allowance for credit losses on our finance receivables and loans, with a resulting negative adjustment to retained earnings. We expect that our consumer automotive loan portfolio will generate the majority of this increase. The amount of the change in the allowance for credit losses will also be impacted by the composition of our portfolio at the adoption date, as well as economic conditions and forecasts at that time. Management created a cross-functional working group to govern the implementation of these amendments, including consideration of model development, data integrity, technology, reporting and disclosure requirements, key accounting interpretations, control environment, and corporate governance. We are in the process of refining and testing the models and procedures that will be used to calculate the credit loss reserves in accordance with these amendments. We performed a limited parallel run during the first quarter of 2019, and will continue to refine and enhance our estimation process with additional parallel testing throughout 2019. Additionally, we do not expect a material allowance for credit losses on our debt securities as a result of the standard based upon the current composition of our portfolio.