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Long-Term Debt (Details 2) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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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 | $ 1,875 us-gaap_InterestExpenseDebtExcludingAmortization / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroThreeSixMember | $ 4,112 us-gaap_InterestExpenseDebtExcludingAmortization / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroThreeSixMember | $ 4,112 us-gaap_InterestExpenseDebtExcludingAmortization / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroThreeSixMember
Year-to-date discount amortization | 5,366 us-gaap_AmortizationOfDebtDiscountPremium / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroThreeSixMember | 7,530 us-gaap_AmortizationOfDebtDiscountPremium / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroThreeSixMember | 6,948 us-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 | 1,839 us-gaap_InterestExpenseDebtExcludingAmortization / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroTwoZeroMember | 3,450 us-gaap_InterestExpenseDebtExcludingAmortization / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroTwoZeroMember | 3,450 us-gaap_InterestExpenseDebtExcludingAmortization / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroTwoZeroMember
Year-to-date discount amortization | $ 1,810 us-gaap_AmortizationOfDebtDiscountPremium / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroTwoZeroMember | $ 3,251 us-gaap_AmortizationOfDebtDiscountPremium / us-gaap_LongtermDebtTypeAxis = gpi_ConvertibleSeniorNotesDueTwoZeroTwoZeroMember | $ 2,966 us-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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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.8) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 8 -Article 5 + Details
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- Definition Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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