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Commitments and Contingencies - (Details) $ in Millions	Dec. 31, 2018 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Lessee rental payment obligations during remaining terms of leases under guarantee agreement	$ 42.1

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- Definition

Guarantor Obligations, Maximum Exposure Inestimable, Minimum Lease Payments Guaranteed

+ References

No definition available.

+ Details

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

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- References

No definition available.

+ Details

Name:	us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration