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Summary of Significant Accounting Policies and Estimates - Goodwill (Details) unit in Millions | 12 Months Ended
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Dec. 31, 2020 unit | Dec. 31, 2019 unit | Dec. 31, 2018 geographic_region
Goodwill [Line Items] |  |  |
Number of geographic regions | geographic_region |  |  | 3
Weight applied to the income approach for determination of the fair value of reporting units |  |  | 80.00%
Weight applied to the market approach for determination of the fair value of reporting units |  |  | 20.00%
Forecast |  |  |
Goodwill [Line Items] |  |  |
Estimated industry sales (in units sold) | unit | 16.7 | 16.8 |

X
- Definition Forecasted seasonally adjusted annual rate of new vehicle sales. + References No definition available. + Details
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Namespace Prefix: | gpi_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of geographic regions + References No definition available. + Details
Name: | gpi_NumberOfGeographicRegions
Namespace Prefix: | gpi_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage weighted income approach to determine fair value of company's reporting units. + References No definition available. + Details
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Namespace Prefix: | gpi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Percentage weighted market approach to determine fair value of company's reporting units. + References No definition available. + Details
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Namespace Prefix: | gpi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_GoodwillLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |