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Asset Impairments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Assets Impairments (Textual) [Abstract]
Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill)	$ 25,600us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill	$ 5,500us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill	$ 5,400us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill	$ 7,000us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill	$ 31,000us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill
Other Asset Impairment Charges					9,200us-gaap_OtherAssetImpairmentCharges
Pre-tax impairment charges of real estate					1,300us-gaap_ImpairmentOfRealEstate
Increase in assumed risk free rate to estimate WACC					2.00%gpi_IncreaseInAssumedRiskFreeRateToEstimateWacc
Pre tax impairment charges of other long lived assets						1,100us-gaap_ImpairmentOfLongLivedAssetsHeldForUse	300us-gaap_ImpairmentOfLongLivedAssetsHeldForUse
No new vehicle sales growth [Member]
Assumption scenario [Line Items]
Additional franchise rights impairment charge					2,000gpi_Additionalfranchiserightsimpairmentcharge / gpi_AssumptionscenarioAxis = gpi_NonewvehiclesalesgrowthMember
WACC increased 200 bps [Member]
Assumption scenario [Line Items]
Additional franchise rights impairment charge					$ 14,000gpi_Additionalfranchiserightsimpairmentcharge / gpi_AssumptionscenarioAxis = gpi_WACCincreased200bpsMember

X

- Definition

Additional franchise rights impairment charge

+ References

No definition available.

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X

- Definition

Increase in assumed risk free rate to estimate WACC.

+ References

No definition available.

+ Details

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X

- Definition

Amount of impairment loss resulting from write-down of assets, excluding financial assets and goodwill, lacking physical substance and having a projected indefinite period of benefit to fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 30

-Section 50

-Paragraph 3

-Subparagraph (b)

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X

- Definition

The aggregate amount of write-downs for impairments recognized during the period for long lived assets held for use (including those held for disposal by means other than sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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- Definition

The charge against earnings in the period to reduce the carrying amount of real property to fair value.

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- Definition

The charge against earnings resulting from the write down of long lived assets other than goodwill due to the difference between the carrying value and lower fair value.

+ References

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