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Commitments and Contingencies (Details Textual) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014
Commitments and Contingencies (Textual) [Abstract]
Rental payment obligations during the remaining terms of these leases	$ 3.2

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- Details

Name:	gpi_CommitmentsAndContingenciesTextualAbstract
Namespace Prefix:	gpi_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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- Definition

Total lease obligation for the period when the Company is the guarantor on a lease, sublets and becomes the secondary guarantor.

+ References

No definition available.

+ Details

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Namespace Prefix:	gpi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant