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Derivative Instruments and Risk Management Activities (Details) (USD $) | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items] |  |
Floor Plan Interest Expense | $ (9,348,000)gpi_FloorPlanInterestExpense | $ (10,913,000)gpi_FloorPlanInterestExpense
Derivatives in cash flow hedging relationship |  |
Interest rate swap contracts | (6,513,000)us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodNetOfTax | (2,520,000)us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodNetOfTax
Increase Decrease In Floor Plan Interest Expense Due To Interest Rate Hedges | 2,700,000 gpi_IncreaseDecreaseInFloorPlanInterestExpenseDueToInterestRateHedges | 2,400,000 gpi_IncreaseDecreaseInFloorPlanInterestExpenseDueToInterestRateHedges
Floorplan Interest Expense [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Floor Plan Interest Expense | (2,708,000)gpi_FloorPlanInterestExpense / us-gaap_IncomeStatementLocationAxis = gpi_FloorplanInterestExpenseMember | (2,429,000)gpi_FloorPlanInterestExpense / us-gaap_IncomeStatementLocationAxis = gpi_FloorplanInterestExpenseMember
Other Interest Expense [Member] |  |
Location of Loss Reclassified from OCI into Statements of Operations |  |
Interest Expense | (439,000)us-gaap_InterestExpense / us-gaap_IncomeStatementLocationAxis = gpi_OtherInterestExpenseMember | (355,000)us-gaap_InterestExpense / us-gaap_IncomeStatementLocationAxis = gpi_OtherInterestExpenseMember
Interest Rate Swap Contracts [Member] |  |
Derivatives in cash flow hedging relationship |  |
Interest rate swap contracts | $ (6,513,000)us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodNetOfTax / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember | $ (2,520,000)us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodNetOfTax / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember

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- Definition Interest expense incurred on borrowings under the floorplan line of the credit facility and arrangements with vehicle manufacturer affiliates for the financing of specific new, used and rental vehicle inventory. Interest expense also includes the net impact of interest rate swap settlements during the period, which are designed to hedge exposure to variable interest rates. + References No definition available. + Details
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- Definition The increase or decrease in floorplan interest expense due to interest rate hedges. + References No definition available. + Details
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- Definition Amount of the cost of borrowed funds accounted for as interest expense. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.9) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 9 -Article 9 + Details
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- Definition 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 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=28358780&loc=d3e637-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=28358780&loc=SL7669646-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6441202&loc=d3e80720-113993 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624171-113959 + Details
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