Summary of Significant Accounting Policies and Estimates Summary of Significant Account Policies and Estimates (Details Textuals)	3 Months Ended				12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Mar. 31, 2013	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Summary of Significant Accounting Policies and Estimates [Line Items]
Variable Interest Entity, Qualitative or Quantitative Information, Ownership Percentage					8.00%
Number of Reportable Segments					3
Change in the historical chargeback results used in determining estimates of future amounts					10.00%
Change in the reserve of future amounts					$ 3,300,000
Restricted cash and short term investments		$ 62,100,000			136,300,000	$ 62,100,000
Inventory cost reduced for interest assistance received from manufacturers		8,800,000			10,300,000	8,800,000
Reduction in new vehicle cost of sales		45,100,000			$ 50,500,000	45,100,000	$ 38,500,000
Used vehicle days supply					33 days
Number of geographic regions					4
Weighted Income approach to determine the fair value of the Company's reporting units					80.00%
Weighted Market approach to determine the fair value of the Company's reporting units					20.00%
Recovery back to normalized levels by 2016					17,800,000
Advertising expenses					$ 74,600,000	73,800,000	59,000,000
Reduction in advertising expenses					$ 17,300,000	16,600,000	24,100,000
Percentage of Company's new vehicle sales volume more than which no other manufacturer accounted for					2.20%
Accounts Receivable, Gross, Current		154,971,000			$ 160,972,000	154,971,000
Interest paid					92,000,000	80,200,000	66,200,000
Income taxes refundable					$ 74,800,000	62,300,000	49,000,000
Percentage of change in loss reserve due to change in actuarial loss rate per employee					10.00%
Change in loss reserve due to change in actuarial loss rate per employee					$ 2,100,000
Limited exposure in no stop loss plan per occurrence					1,000,000
Amount Accrued for Stop Loss Plans					300,000
Amount Accrued for No Stop Loss Plans					20,300,000
Maximum potential loss exposure under all of the Stop Loss Plans before consideration of amounts previously paid or accruals recorded					34,900,000
Remaining potential loss exposure in Stop Loss Plan					13,900,000
Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill)		25,600,000	$ 5,500,000		$ 30,100,000	31,000,000	$ 5,400,000
Toyota [Member]
Summary of Significant Accounting Policies and Estimates [Line Items]
Share in company sales volume					26.40%
Share in company receivable balance					16.30%
BMW [Member]
Summary of Significant Accounting Policies and Estimates [Line Items]
Share in company sales volume					11.60%
Share in company receivable balance					14.80%
Ford [Member]
Summary of Significant Accounting Policies and Estimates [Line Items]
Share in company sales volume					11.40%
Share in company receivable balance					10.00%
Daimler [Member]
Summary of Significant Accounting Policies and Estimates [Line Items]
Share in company sales volume					4.30%
Share in company receivable balance					13.00%
General Motors [Member]
Summary of Significant Accounting Policies and Estimates [Line Items]
Share in company sales volume					7.60%
Share in company receivable balance					16.50%
Nissan [Member]
Summary of Significant Accounting Policies and Estimates [Line Items]
Share in company sales volume					8.30%
Share in company receivable balance					6.70%
Honda [Member]
Summary of Significant Accounting Policies and Estimates [Line Items]
Share in company sales volume					10.90%
Share in company receivable balance					4.20%
Chrysler [Member]
Summary of Significant Accounting Policies and Estimates [Line Items]
Share in company sales volume					4.60%
Share in company receivable balance					2.50%
Hyundai [Member]
Summary of Significant Accounting Policies and Estimates [Line Items]
Share in company sales volume					5.80%
Share in company receivable balance					4.60%
Volkswagen [Member]
Summary of Significant Accounting Policies and Estimates [Line Items]
Share in company sales volume					6.90%
Share in company receivable balance					8.50%
5.00% Senior Note [Member]
Summary of Significant Accounting Policies and Estimates [Line Items]
Unsecured Long-term Debt, Noncurrent		540,100,000			$ 541,252,000	540,100,000
Long-term Debt, Fair Value					545,900,000
5.25% Senior Note [Member]
Summary of Significant Accounting Policies and Estimates [Line Items]
Unsecured Long-term Debt, Noncurrent		0			296,274,000	0
Long-term Debt, Fair Value					297,800,000
Other Real Estate Related Long Term Debt [Member]
Summary of Significant Accounting Policies and Estimates [Line Items]
Other Long-term Debt		$ 358,271,000			$ 311,568,000	$ 358,271,000
Maximum [Member]
Summary of Significant Accounting Policies and Estimates [Line Items]
Interest assistance received related to vehicles sold	139.90%
Minimum [Member]
Summary of Significant Accounting Policies and Estimates [Line Items]
Interest assistance received related to vehicles sold				87.30%
U.S. [Member]
Summary of Significant Accounting Policies and Estimates [Line Items]
Company operates in different states					14
First layer
Summary of Significant Accounting Policies and Estimates [Line Items]
Maximum payout under first layer					$ 0
First layer | Maximum [Member]
Summary of Significant Accounting Policies and Estimates [Line Items]
Per Incident Coverage					10,000,000
Aggregate deductible					30,000,000
Second layer
Summary of Significant Accounting Policies and Estimates [Line Items]
Hail loss insurance deductibles					20,000,000
Insurance deductibles for any weather even other than hail					5,000,000
Second layer | Maximum [Member]
Summary of Significant Accounting Policies and Estimates [Line Items]
Aggregate loss coverage					$ 10,000,000