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Summary of Significant Accounting Policies and Estimates - Cash and Cash Equivalents (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Immediately available funds excluded from cash and cash equivalents	$ 33.7	$ 109.0

X

- Definition

Credit facility floor plan offset.

+ References

No definition available.

+ Details

Name:	gpi_CreditFacilityFloorPlanOffset
Namespace Prefix:	gpi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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- References

No definition available.

+ Details

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