Interim Financial Statements (Policies)	3 Months Ended
Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	The accompanying Unaudited Condensed Consolidated Financial Statements include the accounts of AutoNation, Inc. and its subsidiaries; intercompany accounts and transactions have been eliminated. The accompanying Unaudited Condensed Consolidated Financial Statements have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial information and in accordance with the rules and regulations of the Securities and Exchange Commission (“SEC”). Accordingly, they do not include all of the information and notes required by GAAP for complete financial statements. Additionally, operating results for interim periods are not necessarily indicative of the results that can be expected for a full year. The Unaudited Condensed Consolidated Financial Statements herein should be read in conjunction with our audited Consolidated Financial Statements and notes thereto included within our most recent Annual Report on Form 10-K. These Unaudited Condensed Consolidated Financial Statements reflect, in the opinion of management, all material adjustments (which include only normal recurring adjustments) necessary to fairly state, in all material respects, our financial position and results of operations for the periods presented.
Use of Estimates	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 the reported amounts of revenue and expenses during the reporting period. In preparing these financial statements, management has made its best estimates and judgments of certain amounts included in the financial statements. We base our estimates and judgments on historical experience and other assumptions that we believe are reasonable. However, application of these accounting policies involves the exercise of judgment and use of assumptions as to future uncertainties and, as a result, actual results could differ materially from these estimates. We periodically evaluate estimates and assumptions used in the preparation of the financial statements and make changes on a prospective basis when adjustments are necessary. The critical accounting estimates made in the accompanying Unaudited Condensed Consolidated Financial Statements include certain assumptions related to goodwill, other intangible assets, and accruals for chargebacks against revenue recognized from the sale of finance and insurance products. Other significant accounting estimates include certain assumptions related to long-lived assets, assets held for sale, accruals related to self-insurance programs, certain legal proceedings, and estimated tax liabilities.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Accounting for Leases In February 2016, the Financial Accounting Standards Board (“FASB”) issued an accounting standard update (ASC Topic 842) that amends the accounting guidance on leases. The new standard establishes a right-of-use (ROU) model that requires a lessee to record an ROU asset and a lease liability on the balance sheet for all leases with terms longer than 12 months. Leases will be classified as either finance or operating, with classification affecting the pattern of expense recognition in the income statement. The FASB also subsequently issued amendments to the standard, including providing an additional and optional transition method to adopt the new standard, described below, as well as certain practical expedients related to land easements and lessor accounting. The accounting standard update originally required the use of a modified retrospective approach reflecting the application of the standard to leases existing at, or entered into after, the beginning of the earliest comparative period presented in the financial statements with the option to elect certain practical expedients. A subsequent amendment to the standard provides an additional and optional transition method that allows entities to initially apply the new leases standard at the adoption date and recognize a cumulative effect adjustment to the opening balance of retained earnings in the period of adoption. Consequently, an entity’s reporting for the comparative periods presented in the financial statements in which it adopts the new leases standard will continue to be in accordance with current GAAP (ASC Topic 840) if the optional transition method is elected. The new accounting standard is effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2018. We adopted this accounting standard effective January 1, 2019, using the optional transition method with no restatement of comparative periods. Therefore, the comparative information has not been adjusted and continues to be reported under ASC Topic 840. Our adoption of the new standard did not result in a cumulative effect adjustment to retained earnings. We elected certain practical expedients available under the transition guidance within the new standard, which among other things, allowed us to carry forward the historical lease classification of our existing leases.We did not elect the use-of-hindsight or the practical expedient pertaining to land easements; the latter not being applicable to us. Consequently, on adoption, we recognized additional operating liabilities of $358.1 million and ROU assets of $344.6 million. The new standard also provides practical expedients for an entity’s ongoing accounting. We elected the short-term lease recognition exemption for all leases that qualify. As a result, for those leases that qualify, we will not recognize ROU assets or lease liabilities, and we did not recognize ROU assets or lease liabilities for existing short-term leases of those assets in transition. We also elected the practical expedient to not separate lease and non-lease components of leases for the majority of our classes of underlying assets. See Note 7 for additional information on our leases.
Earnings Per Share	Basic EPS is computed by dividing net income by the weighted average number of common shares outstanding for the period, including outstanding unvested restricted stock awards and vested RSU awards. Diluted EPS is computed by dividing net income by the weighted average number of shares outstanding, noted above, adjusted for the dilutive effect of stock options and unvested RSU awards.
Impairment Of Long-Lived Assets	Nonfinancial assets such as goodwill, other intangible assets, long-lived assets held and used, and right-of-use assets are measured at fair value when there is an indicator of impairment and recorded at fair value only when impairment is recognized or for a business combination. The fair values less costs to sell of long-lived assets held for sale are assessed each reporting period they remain classified as held for sale. Subsequent changes in the held for sale long-lived asset’s fair value less cost to sell (increase or decrease) is reported as an adjustment to its carrying amount, except that the adjusted carrying amount cannot exceed the carrying amount of the long-lived asset at the time it was initially classified as held for sale.

Interim Financial Statements (Policies)

3 Months Ended

Mar. 31, 2019

Organization, Consolidation and Presentation of Financial Statements [Abstract]

The accompanying Unaudited Condensed Consolidated Financial Statements include the accounts of AutoNation, Inc. and its subsidiaries; intercompany accounts and transactions have been eliminated. The accompanying Unaudited Condensed Consolidated Financial Statements have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial information and in accordance with the rules and regulations of the Securities and Exchange Commission (“SEC”). Accordingly, they do not include all of the information and notes required by GAAP for complete financial statements. Additionally, operating results for interim periods are not necessarily indicative of the results that can be expected for a full year. The Unaudited Condensed Consolidated Financial Statements herein should be read in conjunction with our audited Consolidated Financial Statements and notes thereto included within our most recent Annual Report on Form 10-K. These Unaudited Condensed Consolidated Financial Statements reflect, in the opinion of management, all material adjustments (which include only normal recurring adjustments) necessary to fairly state, in all material respects, our financial position and results of operations for the periods presented.

Use of Estimates

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 the reported amounts of revenue and expenses during the reporting period. In preparing these financial statements, management has made its best estimates and judgments of certain amounts included in the financial statements. We base our estimates and judgments on historical experience and other assumptions that we believe are reasonable. However, application of these accounting policies involves the exercise of judgment and use of assumptions as to future uncertainties and, as a result, actual results could differ materially from these estimates. We periodically evaluate estimates and assumptions used in the preparation of the financial statements and make changes on a prospective basis when adjustments are necessary. The critical accounting estimates made in the accompanying Unaudited Condensed Consolidated Financial Statements include certain assumptions related to goodwill, other intangible assets, and accruals for chargebacks against revenue recognized from the sale of finance and insurance products. Other significant accounting estimates include certain assumptions related to long-lived assets, assets held for sale, accruals related to self-insurance programs, certain legal proceedings, and estimated tax liabilities.

Recent Accounting Pronouncements

Accounting for Leases

In February 2016, the Financial Accounting Standards Board (“FASB”) issued an accounting standard update (ASC Topic 842) that amends the accounting guidance on leases. The new standard establishes a right-of-use (ROU) model that requires a lessee to record an ROU asset and a lease liability on the balance sheet for all leases with terms longer than 12 months. Leases will be classified as either finance or operating, with classification affecting the pattern of expense recognition in the income statement. The FASB also subsequently issued amendments to the standard, including providing an additional and optional

transition method to adopt the new standard, described below, as well as certain practical expedients related to land easements and lessor accounting.

The accounting standard update originally required the use of a modified retrospective approach reflecting the application of the standard to leases existing at, or entered into after, the beginning of the earliest comparative period presented in the financial statements with the option to elect certain practical expedients. A subsequent amendment to the standard provides an additional and optional transition method that allows entities to initially apply the new leases standard at the adoption date and recognize a cumulative effect adjustment to the opening balance of retained earnings in the period of adoption. Consequently, an entity’s reporting for the comparative periods presented in the financial statements in which it adopts the new leases standard will continue to be in accordance with current GAAP (ASC Topic 840) if the optional transition method is elected. The new accounting standard is effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2018. We adopted this accounting standard effective January 1, 2019, using the optional transition method with no restatement of comparative periods. Therefore, the comparative information has not been adjusted and continues to be reported under ASC Topic 840. Our adoption of the new standard did not result in a cumulative effect adjustment to retained earnings.

We elected certain practical expedients available under the transition guidance within the new standard, which among other things, allowed us to carry forward the historical lease classification of our existing leases.We did not elect the use-of-hindsight or the practical expedient pertaining to land easements; the latter not being applicable to us. Consequently, on adoption, we recognized additional operating liabilities of $358.1 million and ROU assets of $344.6 million. The new standard also provides practical expedients for an entity’s ongoing accounting. We elected the short-term lease recognition exemption for all leases that qualify. As a result, for those leases that qualify, we will not recognize ROU assets or lease liabilities, and we did not recognize ROU assets or lease liabilities for existing short-term leases of those assets in transition. We also elected the practical expedient to not separate lease and non-lease components of leases for the majority of our classes of underlying assets. See Note 7 for additional information on our leases.

Earnings Per Share

Basic EPS is computed by dividing net income by the weighted average number of common shares outstanding for the period, including outstanding unvested restricted stock awards and vested RSU awards. Diluted EPS is computed by dividing net income by the weighted average number of shares outstanding, noted above, adjusted for the dilutive effect of stock options and unvested RSU awards.

Impairment Of Long-Lived Assets

Nonfinancial assets such as goodwill, other intangible assets, long-lived assets held and used, and right-of-use assets are measured at fair value when there is an indicator of impairment and recorded at fair value only when impairment is recognized or for a business combination. The fair values less costs to sell of long-lived assets held for sale are assessed each reporting period they remain classified as held for sale. Subsequent changes in the held for sale long-lived asset’s fair value less cost to sell (increase or decrease) is reported as an adjustment to its carrying amount, except that the adjusted carrying amount cannot exceed the carrying amount of the long-lived asset at the time it was initially classified as held for sale.