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Derivative Instruments and Risk Management Activities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivatives in cash flow hedging relationship
Interest rate swap contracts	$ (11,153)us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodNetOfTax	$ 6,112us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodNetOfTax	$ (12,724)us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodNetOfTax
Location of Loss Reclassified from OCI into Statements of Operations
Income (Loss) Reclassified from OCI into Statements of Operations	7,094us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax	6,969us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax	7,068us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax
Floorplan Interest Expense [Member]
Location of Loss Reclassified from OCI into Statements of Operations
Income (Loss) Reclassified from OCI into Statements of Operations	(9,837)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax / us-gaap_IncomeStatementLocationAxis = gpi_FloorplanInterestExpenseMember	(9,938)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax / us-gaap_IncomeStatementLocationAxis = gpi_FloorplanInterestExpenseMember	(10,144)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax / us-gaap_IncomeStatementLocationAxis = gpi_FloorplanInterestExpenseMember
Other Interest Expense [Member]
Location of Loss Reclassified from OCI into Statements of Operations
Income (Loss) Reclassified from OCI into Statements of Operations	$ (1,513)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax / us-gaap_IncomeStatementLocationAxis = gpi_OtherInterestExpenseMember	$ (1,213)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax / us-gaap_IncomeStatementLocationAxis = gpi_OtherInterestExpenseMember	$ (1,165)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax / us-gaap_IncomeStatementLocationAxis = gpi_OtherInterestExpenseMember

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Amount after tax of reclassification adjustment from accumulated other comprehensive income of accumulated gain (loss) realized from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's deferred hedging gain (loss).

+ References

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

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-Name Accounting Standards Codification

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Amount after tax of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 220

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-Section 45

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-URI http://asc.fasb.org/extlink&oid=28358780&loc=d3e637-108580



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