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Summary of Significant Accounting Policies and Estimates (Details) (USD $) | 3 Months Ended | 12 Months Ended | 3 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Dec. 31, 2014 | Sep. 30, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Mar. 31, 2012 | Dec. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Mar. 31, 2014 | Jun. 30, 2014 | Jun. 25, 2014 | Sep. 02, 2014
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Secured Long-term Debt, Noncurrent | $ 540,100,000 us-gaap_SecuredLongTermDebt |  | $ 0 us-gaap_SecuredLongTermDebt |  |  | $ 540,100,000 us-gaap_SecuredLongTermDebt | $ 0 us-gaap_SecuredLongTermDebt |  |  |  |  |
Number of Reportable Segments |  |  |  |  |  | 3 us-gaap_NumberOfReportableSegments |  |  |  |  |  |
Assets, Current | 2,035,219,000 us-gaap_AssetsCurrent |  | 1,967,938,000 us-gaap_AssetsCurrent |  |  | 2,035,219,000 us-gaap_AssetsCurrent | 1,967,938,000 us-gaap_AssetsCurrent |  |  |  |  |
Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill) | 25,600,000 us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill | 5,500,000 us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill | 5,400,000 us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill | 7,000,000 us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill |  | 31,000,000 us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill |  |  |  |  |  |
Change in the historical chargeback results used in determining estimates of future amounts | 10.00%gpi_PercentChangeUsedInSensitivityAnalysis |  |  |  |  | 10.00%gpi_PercentChangeUsedInSensitivityAnalysis |  |  |  |  |  |
Changed in the reserve of future amounts | 2,900,000 gpi_SensitivityOfChargeBackInReserve |  |  |  |  | 2,900,000 gpi_SensitivityOfChargeBackInReserve |  |  |  |  |  |
Restricted cash and short term investments | 62,100,000 gpi_CreditFacilityFloorPlanOffset |  | 56,200,000 gpi_CreditFacilityFloorPlanOffset |  |  | 62,100,000 gpi_CreditFacilityFloorPlanOffset | 56,200,000 gpi_CreditFacilityFloorPlanOffset |  |  |  |  |
Inventory cost reduced for interest assistance received from manufacturers | 8,800,000 gpi_ManufacturerFloorplanAssistanceRecognizedAsReductionOfInventoryCost |  | 9,000,000 gpi_ManufacturerFloorplanAssistanceRecognizedAsReductionOfInventoryCost |  |  | 8,800,000 gpi_ManufacturerFloorplanAssistanceRecognizedAsReductionOfInventoryCost | 9,000,000 gpi_ManufacturerFloorplanAssistanceRecognizedAsReductionOfInventoryCost |  |  |  |  |
Reduction in new vehicle cost of sales | 45,100,000 gpi_ManufacturerFloorplanAssistanceRecognizedAsReductionOfCostOfSales |  | 38,500,000 gpi_ManufacturerFloorplanAssistanceRecognizedAsReductionOfCostOfSales | 33,900,000 gpi_ManufacturerFloorplanAssistanceRecognizedAsReductionOfCostOfSales |  | 45,100,000 gpi_ManufacturerFloorplanAssistanceRecognizedAsReductionOfCostOfSales | 38,500,000 gpi_ManufacturerFloorplanAssistanceRecognizedAsReductionOfCostOfSales |  |  |  |  |
Used vehicle days supply |  |  |  |  |  | 33 days |  |  |  |  |  |
Weighted Income approach to determine the fair value of the Company's reporting units | 80.00%gpi_PercentageWeightedIncomeApproachToDetermineFairValueOfCompanysReportingUnits |  |  |  |  | 80.00%gpi_PercentageWeightedIncomeApproachToDetermineFairValueOfCompanysReportingUnits |  |  |  |  |  |
Weighted Market approach to determine the fair value of the Company's reporting units | 20.00%gpi_PercentageWeightedMarketApproachToDetermineFairValueOfCompanysReportingUnits |  |  |  |  | 20.00%gpi_PercentageWeightedMarketApproachToDetermineFairValueOfCompanysReportingUnits |  |  |  |  |  |
Recovery back to normalized levels by 2016 | 17,000,000 gpi_ForecastedSeasonallyAdjustedAnnualRateOfNewVehicleSales |  |  |  |  | 17,000,000 gpi_ForecastedSeasonallyAdjustedAnnualRateOfNewVehicleSales |  |  |  |  |  |
Advertising expenses |  |  |  |  | 54,100,000 us-gaap_AdvertisingExpense | 73,800,000 us-gaap_AdvertisingExpense | 59,000,000 us-gaap_AdvertisingExpense |  |  |  |  |
Reduction in advertising expenses |  |  |  |  | 23,700,000 gpi_ManufacturerAdvertisingAssistanceRecognizedAsReductionOfAdvertisingExpense | 16,600,000 gpi_ManufacturerAdvertisingAssistanceRecognizedAsReductionOfAdvertisingExpense | 24,100,000 gpi_ManufacturerAdvertisingAssistanceRecognizedAsReductionOfAdvertisingExpense |  |  |  |  |
Percentage of Company's new vehicle sales volume more then which no other manufacturer accounted for |  |  |  |  |  | 3.00%gpi_PercentageOfCompanysNewVehicleSalesVolumeMoreThenWhichNoOtherManufacturerAccountedFor |  |  |  |  |  |
Accounts Receivable, Gross, Current | 154,971,000 us-gaap_AccountsReceivableGrossCurrent |  | 137,463,000 us-gaap_AccountsReceivableGrossCurrent |  |  | 154,971,000 us-gaap_AccountsReceivableGrossCurrent | 137,463,000 us-gaap_AccountsReceivableGrossCurrent |  |  |  |  |
Interest paid |  |  |  |  | 55,800,000 us-gaap_InterestPaid | 80,200,000 us-gaap_InterestPaid | 66,200,000 us-gaap_InterestPaid |  |  |  |  |
Income taxes refundable |  |  |  |  | 38,600,000 us-gaap_ProceedsFromIncomeTaxRefunds | 62,300,000 us-gaap_ProceedsFromIncomeTaxRefunds | 49,000,000 us-gaap_ProceedsFromIncomeTaxRefunds |  |  |  |  |
Percentage of change in loss reserve due to change in actuarial loss rate per employee | 10.00%gpi_PercentageOfChangeInLossReserveDueToChangeInActuarialLossRatePerEmployee |  |  |  |  | 10.00%gpi_PercentageOfChangeInLossReserveDueToChangeInActuarialLossRatePerEmployee |  |  |  |  |  |
Change in loss reserve due to change in actuarial loss rate per employee | 2,100,000 gpi_SensitivityInWorkersCompensationAndGeneralLiabilityReserve |  |  |  |  | 2,100,000 gpi_SensitivityInWorkersCompensationAndGeneralLiabilityReserve |  |  |  |  |  |
Limited exposure in no stop loss plan per occurrence |  |  |  |  |  | 1,000,000 gpi_LimitedExposureInNoStopLossPlanPerOccurrence |  |  |  |  |  |
Amount Accrued for Stop Loss Plans | 500,000 gpi_AmountAccruedforStopLossPlans |  |  |  |  | 500,000 gpi_AmountAccruedforStopLossPlans |  |  |  |  |  |
Amount Accrued for No Stop Loss Plans | 20,300,000 gpi_AmountAccruedforNoStopLossPlans |  |  |  |  | 20,300,000 gpi_AmountAccruedforNoStopLossPlans |  |  |  |  |  |
Maximum potential loss exposure under all of the Stop Loss Plans before consideration of amounts previously paid or accruals recorded | 34,900,000 gpi_MaximumPotentialLossExposureUnderAllOfStopLossPlansBeforeConsiderationOfAmountsPreviouslyPaidOrAccrualsRecorded |  |  |  |  | 34,900,000 gpi_MaximumPotentialLossExposureUnderAllOfStopLossPlansBeforeConsiderationOfAmountsPreviouslyPaidOrAccrualsRecorded |  |  |  |  |  |
Remaining potential loss exposure in Stop Loss Plan | 13,800,000 gpi_RemainingPotentialLossExposureInStopLossPlan |  |  |  |  | 13,800,000 gpi_RemainingPotentialLossExposureInStopLossPlan |  |  |  |  |  |
Total assets | 4,141,492,000 us-gaap_Assets |  | 3,819,478,000 us-gaap_Assets |  |  | 4,141,492,000 us-gaap_Assets | 3,819,478,000 us-gaap_Assets |  |  |  |  |
Liabilities, Current | 1,922,199,000 us-gaap_LiabilitiesCurrent |  | 1,865,176,000 us-gaap_LiabilitiesCurrent |  |  | 1,922,199,000 us-gaap_LiabilitiesCurrent | 1,865,176,000 us-gaap_LiabilitiesCurrent |  |  |  |  |
Toyota [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share in company sales volume |  |  |  |  |  | 26.70%gpi_PercentageOfNewVehicleSalesVolume / dei_LegalEntityAxis = gpi_ToyotaMember |  |  |  |  |  |
Share in company receivable balance | 17.70%gpi_PercentageOfAmountsDueFromManufacturers / dei_LegalEntityAxis = gpi_ToyotaMember |  |  |  |  | 17.70%gpi_PercentageOfAmountsDueFromManufacturers / dei_LegalEntityAxis = gpi_ToyotaMember |  |  |  |  |  |
BMW [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share in company sales volume |  |  |  |  |  | 11.50%gpi_PercentageOfNewVehicleSalesVolume / dei_LegalEntityAxis = gpi_BmwMember |  |  |  |  |  |
Share in company receivable balance | 14.30%gpi_PercentageOfAmountsDueFromManufacturers / dei_LegalEntityAxis = gpi_BmwMember |  |  |  |  | 14.30%gpi_PercentageOfAmountsDueFromManufacturers / dei_LegalEntityAxis = gpi_BmwMember |  |  |  |  |  |
Ford [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share in company sales volume |  |  |  |  |  | 10.90%gpi_PercentageOfNewVehicleSalesVolume / dei_LegalEntityAxis = gpi_FordMember |  |  |  |  |  |
Share in company receivable balance | 8.80%gpi_PercentageOfAmountsDueFromManufacturers / dei_LegalEntityAxis = gpi_FordMember |  |  |  |  | 8.80%gpi_PercentageOfAmountsDueFromManufacturers / dei_LegalEntityAxis = gpi_FordMember |  |  |  |  |  |
Daimler [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share in company sales volume |  |  |  |  |  | 4.50%gpi_PercentageOfNewVehicleSalesVolume / dei_LegalEntityAxis = gpi_DaimlerMember |  |  |  |  |  |
Share in company receivable balance | 15.30%gpi_PercentageOfAmountsDueFromManufacturers / dei_LegalEntityAxis = gpi_DaimlerMember |  |  |  |  | 15.30%gpi_PercentageOfAmountsDueFromManufacturers / dei_LegalEntityAxis = gpi_DaimlerMember |  |  |  |  |  |
General Motors [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share in company sales volume |  |  |  |  |  | 6.10%gpi_PercentageOfNewVehicleSalesVolume / dei_LegalEntityAxis = gpi_GeneralMotorsMember |  |  |  |  |  |
Share in company receivable balance | 16.00%gpi_PercentageOfAmountsDueFromManufacturers / dei_LegalEntityAxis = gpi_GeneralMotorsMember |  |  |  |  | 16.00%gpi_PercentageOfAmountsDueFromManufacturers / dei_LegalEntityAxis = gpi_GeneralMotorsMember |  |  |  |  |  |
Nissan [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share in company sales volume |  |  |  |  |  | 9.40%gpi_PercentageOfNewVehicleSalesVolume / dei_LegalEntityAxis = gpi_NissanMember |  |  |  |  |  |
Share in company receivable balance | 6.70%gpi_PercentageOfAmountsDueFromManufacturers / dei_LegalEntityAxis = gpi_NissanMember |  |  |  |  | 6.70%gpi_PercentageOfAmountsDueFromManufacturers / dei_LegalEntityAxis = gpi_NissanMember |  |  |  |  |  |
Honda [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share in company sales volume |  |  |  |  |  | 11.30%gpi_PercentageOfNewVehicleSalesVolume / dei_LegalEntityAxis = gpi_HondaMember |  |  |  |  |  |
Share in company receivable balance | 4.50%gpi_PercentageOfAmountsDueFromManufacturers / dei_LegalEntityAxis = gpi_HondaMember |  |  |  |  | 4.50%gpi_PercentageOfAmountsDueFromManufacturers / dei_LegalEntityAxis = gpi_HondaMember |  |  |  |  |  |
Chrysler [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share in company sales volume |  |  |  |  |  | 4.40%gpi_PercentageOfNewVehicleSalesVolume / dei_LegalEntityAxis = gpi_ChryslerMember |  |  |  |  |  |
Share in company receivable balance | 3.20%gpi_PercentageOfAmountsDueFromManufacturers / dei_LegalEntityAxis = gpi_ChryslerMember |  |  |  |  | 3.20%gpi_PercentageOfAmountsDueFromManufacturers / dei_LegalEntityAxis = gpi_ChryslerMember |  |  |  |  |  |
Hyundai [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share in company sales volume |  |  |  |  |  | 5.50%gpi_PercentageOfNewVehicleSalesVolume / dei_LegalEntityAxis = gpi_HyundaiMember |  |  |  |  |  |
Share in company receivable balance | 3.80%gpi_PercentageOfAmountsDueFromManufacturers / dei_LegalEntityAxis = gpi_HyundaiMember |  |  |  |  | 3.80%gpi_PercentageOfAmountsDueFromManufacturers / dei_LegalEntityAxis = gpi_HyundaiMember |  |  |  |  |  |
Volkswagen [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share in company sales volume |  |  |  |  |  | 6.40%gpi_PercentageOfNewVehicleSalesVolume / dei_LegalEntityAxis = gpi_VolkswagenMember |  |  |  |  |  |
Share in company receivable balance | 6.50%gpi_PercentageOfAmountsDueFromManufacturers / dei_LegalEntityAxis = gpi_VolkswagenMember |  |  |  |  | 6.50%gpi_PercentageOfAmountsDueFromManufacturers / dei_LegalEntityAxis = gpi_VolkswagenMember |  |  |  |  |  |
3.00% Convertible Notes due 2020 |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Secured Long-term Debt, Noncurrent | 0 us-gaap_SecuredLongTermDebt / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroTwoZeroMember |  | 84,305,000 us-gaap_SecuredLongTermDebt / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroTwoZeroMember |  |  | 0 us-gaap_SecuredLongTermDebt / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroTwoZeroMember | 84,305,000 us-gaap_SecuredLongTermDebt / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroTwoZeroMember |  |  |  |  |
Principal amount of Notes |  |  |  |  |  |  |  |  |  | 22,600,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroTwoZeroMember | 115,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroTwoZeroMember |
2.25% Convertible Notes due 2036 |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Secured Long-term Debt, Noncurrent | 0 us-gaap_SecuredLongTermDebt / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroThreeSixMember |  | 160,334,000 us-gaap_SecuredLongTermDebt / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroThreeSixMember |  |  | 0 us-gaap_SecuredLongTermDebt / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroThreeSixMember | 160,334,000 us-gaap_SecuredLongTermDebt / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroThreeSixMember |  |  |  |  |
Principal amount of Notes |  | 182,800,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroThreeSixMember |  |  |  |  |  |  |  |  |  | 182,500,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroThreeSixMember
Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Interest assistance received related to vehicles sold |  |  |  |  |  |  |  | 117.70%gpi_InterestAssistanceReceivedRelatedToVehiclesSold / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |  |
Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Interest assistance received related to vehicles sold |  |  |  |  |  |  |  |  | 87.30%gpi_InterestAssistanceReceivedRelatedToVehiclesSold / us-gaap_RangeAxis = us-gaap_MinimumMember |  |  |
UNITED STATES |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Company operates in different states | 14 us-gaap_NumberOfStatesInWhichEntityOperates / us-gaap_StatementGeographicalAxis = country_US |  |  |  |  | 14 us-gaap_NumberOfStatesInWhichEntityOperates / us-gaap_StatementGeographicalAxis = country_US |  |  |  |  |  |
Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Assets, Current | 19,049,000 us-gaap_AssetsCurrent / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 24,170,000 us-gaap_AssetsCurrent / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  |  | 19,049,000 us-gaap_AssetsCurrent / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | 24,170,000 us-gaap_AssetsCurrent / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  |  |  |  |
Long-Lived Assets | 31,783,000 us-gaap_NoncurrentAssets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 71,033,000 us-gaap_NoncurrentAssets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  |  | 31,783,000 us-gaap_NoncurrentAssets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | 71,033,000 us-gaap_NoncurrentAssets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  |  |  |  |
Total assets | 50,832,000 us-gaap_Assets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 95,203,000 us-gaap_Assets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  |  | 50,832,000 us-gaap_Assets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | 95,203,000 us-gaap_Assets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  |  |  |  |
Liabilities, Current | 16,374,000 us-gaap_LiabilitiesCurrent / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 21,653,000 us-gaap_LiabilitiesCurrent / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  |  | 16,374,000 us-gaap_LiabilitiesCurrent / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | 21,653,000 us-gaap_LiabilitiesCurrent / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  |  |  |  |
Liabilities, Noncurrent | 15,955,000 us-gaap_LiabilitiesNoncurrent / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 25,374,000 us-gaap_LiabilitiesNoncurrent / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  |  | 15,955,000 us-gaap_LiabilitiesNoncurrent / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | 25,374,000 us-gaap_LiabilitiesNoncurrent / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  |  |  |  |
Liabilities | 32,329,000 us-gaap_Liabilities / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 47,027,000 us-gaap_Liabilities / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  |  | 32,329,000 us-gaap_Liabilities / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | 47,027,000 us-gaap_Liabilities / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  |  |  |  |
First layer |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Maximum payout under first layer |  |  |  |  |  | 0 gpi_Maximumpayoutunderfirstlayer / gpi_LayerofcoverageAxis = gpi_FirstlayerMember |  |  |  |  |  |
First layer | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Per Incident Coverage | 10,000,000 gpi_PerIncidentCoverage / gpi_LayerofcoverageAxis = gpi_FirstlayerMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |  |  | 10,000,000 gpi_PerIncidentCoverage / gpi_LayerofcoverageAxis = gpi_FirstlayerMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |  |  |  |
Aggregate deductible |  |  |  |  |  | 30,000,000 gpi_Aggregatedeductible / gpi_LayerofcoverageAxis = gpi_FirstlayerMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |  |  |  |
Second layer |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Hail loss insurance deductibles |  |  |  |  |  | 20,000,000 gpi_Haillossinsurancedeductibles / gpi_LayerofcoverageAxis = gpi_SecondlayerMember |  |  |  |  |  |
Insurance deductibles for any weather even other than hail |  |  |  |  |  | 5,000,000 gpi_Insurancedeductiblesforanyweatherevenotherthanhail / gpi_LayerofcoverageAxis = gpi_SecondlayerMember |  |  |  |  |  |
Second layer | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies and Estimates [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Aggregate loss coverage | $ 10,000,000 gpi_Aggregatelosscoverage / gpi_LayerofcoverageAxis = gpi_SecondlayerMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |  |  | $ 10,000,000 gpi_Aggregatelosscoverage / gpi_LayerofcoverageAxis = gpi_SecondlayerMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |  |  |  |

X
- Definition Aggregate deductible + References No definition available. + Details
Name: | gpi_Aggregatedeductible
Namespace Prefix: | gpi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Aggregate loss coverage + References No definition available. + Details
Name: | gpi_Aggregatelosscoverage
Namespace Prefix: | gpi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount Accrued for No Stop Loss Plans + References No definition available. + Details
Name: | gpi_AmountAccruedforNoStopLossPlans
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