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Employee Savings Plans (Details Textual) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Employee Savings Plans (Textual) [Abstract] |  |
Deferred compensation plan related to participants | $ 56.6 | $ 49.0
Matching contribution paid | $ 5.1 | $ 5.4

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- Definition Employee savings plans. + References No definition available. + Details
Name: | gpi_EmployeeSavingsPlansTextualAbstract
Namespace Prefix: | gpi_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The amount of payment into the fund established for purposes of making future disbursements to the individual in accordance with the terms and understanding of the deferred compensation arrangement. + References No definition available. + Details
Name: | us-gaap_DeferredCompensationArrangementWithIndividualEmployerContribution
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Aggregate carrying value as of the balance sheet date of the liabilities for all deferred compensation arrangements payable beyond one year (or the operating cycle, if longer). + References No definition available. + Details
Name: | us-gaap_DeferredCompensationLiabilityClassifiedNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant