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Business and Summary of Significant Accounting Policies - Statements of Cash Flows (Details)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Percent of value of the vehicle financed by the company (up to)	85.00%

X

- Definition

Value of used vehicle inventory financed

+ References

No definition available.

+ Details

Name:	gpi_ValueOfUsedVehicleInventoryFinanced
Namespace Prefix:	gpi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- References

No definition available.

+ Details

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