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Summary of Significant Accounting Policies and Estimates - Income Taxes (Details)	Dec. 31, 2018 state
U.S.
Income Taxes [Line Items]
Number of states in which the entity operates	15

X

- Definition

[Line Items] for Income Taxes [Table]

+ References

No definition available.

+ Details

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

X

- Definition

The number of states the entity operates in as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfStatesInWhichEntityOperates
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

X

- Details

Name:	srt_StatementGeographicalAxis=country_US
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: