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Condensed Consolidated Financial Information - Condensed Consolidated Balance Sheets Parenthetical (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Condensed Financial Information Disclosure [Abstract]
Offset account related to floorplan notes payable	$ 106.8	$ 33.6
FMCC offset	$ 4.1	$ 0.1

X

- Definition

FMCC Offset

+ References

No definition available.

+ Details

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

X

- Definition

Floorplan offset.

+ References

No definition available.

+ Details

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

X

- References

No definition available.

+ Details

Name:	srt_CondensedFinancialInformationOfParentCompanyOnlyDisclosureAbstract
Namespace Prefix:	srt_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration