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Employee Savings Plans (Details Textual) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Employee Savings Plans (Textual) [Abstract]
Deferred compensation plan related to participants	$ 34.0us-gaap_DeferredCompensationLiabilityClassifiedNoncurrent	$ 30.5us-gaap_DeferredCompensationLiabilityClassifiedNoncurrent
Matching contribution paid	$ 4.6us-gaap_DeferredCompensationArrangementWithIndividualEmployerContribution	$ 4.1us-gaap_DeferredCompensationArrangementWithIndividualEmployerContribution

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- 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