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Summary of Significant Accounting Policies and Estimates Summary of Significant Account Policies and Estimates (Details Textuals) | 3 Months Ended | 12 Months Ended |  |  |
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Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) | Dec. 31, 2016 USD ($) | Sep. 30, 2016 USD ($) | Mar. 31, 2016 USD ($) | Mar. 31, 2015 | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Value of used vehicle inventory financed |  |  |  |  |  |  | 85.00% |  |  |  |  |
Number of Reportable Segments |  |  |  |  |  |  | 3 |  |  |  |  |
Change in the historical chargeback results used in determining estimates of future amounts | 10.00% |  |  |  |  |  | 10.00% |  |  |  |  |
Change in the reserve of future amounts | $ 5,000,000 |  |  |  |  |  | $ 5,000,000 |  |  |  |  |
Restricted cash and short term investments | 109,000,000 |  | $ 85,100,000 |  |  |  | 109,000,000 | $ 85,100,000 |  |  |  |
Inventory cost reduced for interest assistance received from manufacturers | 9,100,000 |  | 9,600,000 |  |  |  | 9,100,000 | 9,600,000 |  |  |  |
Reduction in new vehicle cost of sales | $ 48,900,000 |  | 49,200,000 |  |  |  | $ 48,900,000 | $ 49,200,000 | $ 50,500,000 |  |  |
Number of geographic regions |  |  |  |  |  |  | 3 | 4 |  |  |  |
Weighted Income approach to determine the fair value of the Company's reporting units | 80.00% |  |  |  |  |  | 80.00% |  |  |  |  |
Weighted Market approach to determine the fair value of the Company's reporting units | 20.00% |  |  |  |  |  | 20.00% |  |  |  |  |
Advertising expenses |  |  |  |  |  |  | $ 74,100,000 | $ 75,300,000 | 74,600,000 |  |  |
Reduction in advertising expenses |  |  |  |  |  |  | $ 15,300,000 | 16,700,000 | 17,300,000 |  |  |
Percentage of Company's new vehicle sales volume more than which no other manufacturer accounted for |  |  |  |  |  |  | 2.90% |  |  |  |  |
Accounts Receivable, Gross, Current | $ 191,749,000 |  | 176,260,000 |  |  |  | $ 191,749,000 | 176,260,000 |  |  |  |
Interest paid |  |  |  |  |  |  | 117,100,000 | 109,300,000 | 92,000,000 |  |  |
Income taxes refundable |  |  |  |  |  |  | $ 61,000,000 | 56,900,000 | 74,800,000 |  |  |
Percentage of change in loss reserve due to change in actuarial loss rate per employee | 10.00% |  |  |  |  |  | 10.00% |  |  |  |  |
Change in loss reserve due to change in actuarial loss rate per employee | $ 2,100,000 |  |  |  |  |  | $ 2,100,000 |  |  |  |  |
Maximum payout under first layer |  |  |  |  |  |  | 0 |  |  |  |  |
Limited exposure in no stop loss plan per occurrence |  |  |  |  |  |  | 1,000,000 |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims |  |  |  |  |  |  | 20,500,000 |  |  |  |  |
Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill) | $ 9,800,000 | $ 9,500,000 | 19,200,000 | $ 10,600,000 | $ 200,000 |  | $ 19,300,000 | 30,000,000 | $ 30,100,000 |  |  |
Used vehicle days supply |  |  |  |  |  |  | 39 days |  |  |  |  |
FCA US [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share in company sales volume |  |  |  |  |  |  | 3.90% |  |  |  |  |
Share in company receivable balance | 3.40% |  |  |  |  |  | 3.40% |  |  |  |  |
Toyota [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share in company sales volume |  |  |  |  |  |  | 25.30% |  |  |  |  |
Share in company receivable balance | 16.60% |  |  |  |  |  | 16.60% |  |  |  |  |
BMW [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share in company sales volume |  |  |  |  |  |  | 12.70% |  |  |  |  |
Share in company receivable balance | 14.10% |  |  |  |  |  | 14.10% |  |  |  |  |
Ford [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share in company sales volume |  |  |  |  |  |  | 11.50% |  |  |  |  |
Share in company receivable balance | 13.50% |  |  |  |  |  | 13.50% |  |  |  |  |
Daimler [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share in company sales volume |  |  |  |  |  |  | 4.00% |  |  |  |  |
Share in company receivable balance | 9.00% |  |  |  |  |  | 9.00% |  |  |  |  |
General Motors [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share in company sales volume |  |  |  |  |  |  | 6.20% |  |  |  |  |
Share in company receivable balance | 14.00% |  |  |  |  |  | 14.00% |  |  |  |  |
Nissan [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share in company sales volume |  |  |  |  |  |  | 7.40% |  |  |  |  |
Share in company receivable balance | 5.00% |  |  |  |  |  | 5.00% |  |  |  |  |
Honda [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share in company sales volume |  |  |  |  |  |  | 9.20% |  |  |  |  |
Share in company receivable balance | 3.20% |  |  |  |  |  | 3.20% |  |  |  |  |
Hyundai [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share in company sales volume |  |  |  |  |  |  | 3.90% |  |  |  |  |
Share in company receivable balance | 3.80% |  |  |  |  |  | 3.80% |  |  |  |  |
Volkswagen [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share in company sales volume |  |  |  |  |  |  | 13.00% |  |  |  |  |
Share in company receivable balance | 11.20% |  |  |  |  |  | 11.20% |  |  |  |  |
5.00% Senior Note [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Unsecured Long-term Debt, Noncurrent | $ 542,063,000 |  | 540,465,000 |  |  |  | $ 542,063,000 | 540,465,000 |  |  |  |
Long-term Debt, Fair Value | 567,900,000 |  | 548,400,000 |  |  |  | 567,900,000 | 548,400,000 |  |  |  |
5.25% Senior Note [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Unsecured Long-term Debt, Noncurrent | 296,151,000 |  | 295,591,000 |  |  |  | 296,151,000 | 295,591,000 |  |  |  |
Long-term Debt, Fair Value | 310,900,000 |  | 297,000,000 |  |  |  | 310,900,000 | 297,000,000 |  |  |  |
Real Estate Related and Other Long Term Debt [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Other Long-term Debt | $ 440,845,000 |  | 385,358,000 |  |  |  | 440,845,000 | 385,358,000 |  |  |  |
Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Interest assistance received related to vehicles sold |  |  |  | 139.90% |  |  |  |  |  |  |  |
Per Occurrence Company Deductible |  |  |  |  |  |  | 10,000,000 |  |  |  |  |
Aggregate deductible |  |  |  |  |  |  | $ 30,000,000 |  |  |  |  |
Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Interest assistance received related to vehicles sold |  |  |  |  |  | 88.00% |  |  |  |  |  |
U.S. |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Company operates in different states | 15 |  |  |  |  |  | 15 |  |  |  |  |
Amounts Due From Manufacturers [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Accounts Receivable, Gross, Current | $ 109,599,000 |  | $ 95,754,000 |  |  |  | $ 109,599,000 | $ 95,754,000 |  |  |  |
Scenario, Forecast [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Recovery back to normalized levels by 2016 |  |  |  |  |  |  |  |  |  | 16,500,000 | 16,700,000 | 16,900,000

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