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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
--- | --- | ---
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 |  |

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Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | gpi_LayerofcoverageAxis=gpi_FirstlayerMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | gpi_LayerofcoverageAxis=gpi_SecondlayerMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |