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Long-Term Debt (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
2.25% Convertible Notes due 2036
Contractual interest expense and the discount amortization
Year-to-date contractual interest expense	$ 1,875us-gaap_InterestExpenseDebtExcludingAmortization / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroThreeSixMember	$ 4,112us-gaap_InterestExpenseDebtExcludingAmortization / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroThreeSixMember	$ 4,112us-gaap_InterestExpenseDebtExcludingAmortization / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroThreeSixMember
Year-to-date discount amortization	5,366us-gaap_AmortizationOfDebtDiscountPremium / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroThreeSixMember	7,530us-gaap_AmortizationOfDebtDiscountPremium / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroThreeSixMember	6,948us-gaap_AmortizationOfDebtDiscountPremium / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroThreeSixMember
Debt Instrument, Interest Rate, Effective Percentage	7.70%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroThreeSixMember	7.70%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroThreeSixMember	7.70%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroThreeSixMember
3.00% Convertible Notes due 2020
Contractual interest expense and the discount amortization
Year-to-date contractual interest expense	1,839us-gaap_InterestExpenseDebtExcludingAmortization / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroTwoZeroMember	3,450us-gaap_InterestExpenseDebtExcludingAmortization / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroTwoZeroMember	3,450us-gaap_InterestExpenseDebtExcludingAmortization / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroTwoZeroMember
Year-to-date discount amortization	$ 1,810us-gaap_AmortizationOfDebtDiscountPremium / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroTwoZeroMember	$ 3,251us-gaap_AmortizationOfDebtDiscountPremium / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroTwoZeroMember	$ 2,966us-gaap_AmortizationOfDebtDiscountPremium / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroTwoZeroMember
Debt Instrument, Interest Rate, Effective Percentage	8.60%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroTwoZeroMember	8.60%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroTwoZeroMember	8.60%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroTwoZeroMember

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Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Represents the portion of interest incurred in the period on debt arrangements that was charged against earnings, excluding amortization of debt discount (premium) and financing costs.

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