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Income Taxes (Details Textual) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Valuation Allowance [Line Items]        
Deferred Tax Liabilities, Net, Noncurrent $ 202,100,000us-gaap_DeferredTaxLiabilitiesNoncurrent $ 216,200,000us-gaap_DeferredTaxLiabilitiesNoncurrent    
Income Taxes (Textual) [Abstract]        
U.S. federal statutory corporate tax rate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate  
Provision for income taxes 71,396,000us-gaap_IncomeTaxExpenseBenefit 77,903,000us-gaap_IncomeTaxExpenseBenefit 60,526,000us-gaap_IncomeTaxExpenseBenefit  
Effective tax rate 43.40%us-gaap_EffectiveIncomeTaxRateContinuingOperations 40.60%us-gaap_EffectiveIncomeTaxRateContinuingOperations 37.70%us-gaap_EffectiveIncomeTaxRateContinuingOperations 37.60%us-gaap_EffectiveIncomeTaxRateContinuingOperations
Long-term deferred tax assets 63,700,000us-gaap_DeferredTaxAssetsNetNoncurrent 64,900,000us-gaap_DeferredTaxAssetsNetNoncurrent    
Number Of Dealerships Acquired     16gpi_NumberOfDealershipsAcquired  
Provision for deferred taxes on undistributed earnings 16,700,000gpi_ProvisionForDeferredTaxesOnUndistributedEarnings      
Foreign earnings potential tax 4,700,000gpi_Foreignearningspotentialtax      
No unrecognized tax benefits 0us-gaap_UnrecognizedTaxBenefits 0us-gaap_UnrecognizedTaxBenefits    
2.25% Notes [Member]        
Valuation Allowance [Line Items]        
Debt interest rate 2.25%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_LongtermDebtTypeAxis
= gpi_TwoPointTwoFivePercentNotesMember		      
3.00% Notes [Member]        
Valuation Allowance [Line Items]        
Debt interest rate 3.00%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_LongtermDebtTypeAxis
= gpi_ThreePointZeroZeroPercentNotesMember		      
UNITED KINGDOM        
Income Taxes (Textual) [Abstract]        
Number Of Dealerships Acquired 3gpi_NumberOfDealershipsAcquired
/ gpi_CountriesAndStatesAxis
= country_GB		 4gpi_NumberOfDealershipsAcquired
/ gpi_CountriesAndStatesAxis
= country_GB		 6gpi_NumberOfDealershipsAcquired
/ gpi_CountriesAndStatesAxis
= country_GB		  
Foreign [Member]        
Valuation Allowance [Line Items]        
Deferred Tax Liabilities, Net, Noncurrent 9,600,000us-gaap_DeferredTaxLiabilitiesNoncurrent
/ us-gaap_StatementGeographicalAxis
= gpi_ForeignMember		 27,500,000us-gaap_DeferredTaxLiabilitiesNoncurrent
/ us-gaap_StatementGeographicalAxis
= gpi_ForeignMember		    
Operating Loss Carryforwards 65,500,000us-gaap_OperatingLossCarryforwards
/ us-gaap_StatementGeographicalAxis
= gpi_ForeignMember		      
State [Member]        
Valuation Allowance [Line Items]        
Operating Loss Carryforwards $ 244,200,000us-gaap_OperatingLossCarryforwards
/ us-gaap_StatementGeographicalAxis
= gpi_StateMember		      
Minimum [Member]        
Valuation Allowance [Line Items]        
Operating Loss Carryforwards, Expiration Dates Dec. 31, 2015      
Maximum [Member]        
Valuation Allowance [Line Items]        
Operating Loss Carryforwards, Expiration Dates Dec. 31, 2034