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Summary of Significant Accounting Policies and Estimates - Income Taxes (Details) | Dec. 31, 2018 state
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U.S. |
Income Taxes [Line Items] |
Number of states in which the entity operates | 15

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- Definition [Line Items] for Income Taxes [Table] + References No definition available. + Details
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Namespace Prefix: | gpi_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The number of states the entity operates in as of the balance sheet date. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | srt_StatementGeographicalAxis=country_US
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |