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Stock-Based Compensation Plans (Details Textual) (USD $) In Thousands, except Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2012 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Stock-based Compensation Plans (Textual) [Abstract] |  |  |  |
Unrecognized compensation cost related to stock based compensation arrangements |  | $ 41,400 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized |  |
Weighted average period for recognition of cost |  | 3 years 5 months 29 days |  |
Additional Stock-based Compensation Plans (Textual) [Abstract] |  |  |  |
Stock-based compensation |  | 16,012 us-gaap_ShareBasedCompensation | 13,899 us-gaap_ShareBasedCompensation | 11,931 us-gaap_ShareBasedCompensation
Income tax benefit for stock based compensation arrangements | 3,200 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense | 4,400 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense | 3,700 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense |
Cash received from option exercises and employee stock purchase plan purchases | 5,700 gpi_ProceedsFromOptionExercisesAndEmployeeStockPurchasePlanPurchases | 6,000 gpi_ProceedsFromOptionExercisesAndEmployeeStockPurchasePlanPurchases | 5,600 gpi_ProceedsFromOptionExercisesAndEmployeeStockPurchasePlanPurchases |
Tax benefit realized for the tax deductions | 3,000 gpi_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptionsAndVestingOfRestrictedShares | 1,841 gpi_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptionsAndVestingOfRestrictedShares | 3,000 gpi_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptionsAndVestingOfRestrictedShares |
Increase in financing activities and decrease in operating activities due to tax benefit relating to excess stock based compensation deductions |  | 1,841 us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities | 2,993 us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities | 2,875 us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities
2007 Long Term Incentive Plan [Member] |  |  |  |
Stock-based Compensation Plans (Textual) [Abstract] |  |  |  |
Maximum number of shares available for issuance under the plan |  | 7,500,000 gpi_MaximumNumberOfSharesAvailableForIssuanceUnderPlan / us-gaap_AwardTypeAxis = gpi_A2007LongTermIncentivePlanMember |  |
2014 Long Term Incentive Plan [Member] |  |  |  |
Stock-based Compensation Plans (Textual) [Abstract] |  |  |  |
Maximum number of shares available for issuance under the plan |  | 1,200,000 gpi_MaximumNumberOfSharesAvailableForIssuanceUnderPlan / us-gaap_AwardTypeAxis = gpi_A2014LongTermIncentivePlanMember |  |
Expiry date for stock options grant |  | May 21, 2024 |  |
Shares available for granted under the incentive plan |  | 1,711,034 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / us-gaap_AwardTypeAxis = gpi_A2014LongTermIncentivePlanMember |  |
Employee Stock Purchase Plan [Member] |  |  |  |
Stock-based Compensation Plans (Textual) [Abstract] |  |  |  |
Expiry date for stock options grant |  | Mar. 06, 2016 |  |
Shares authorized under Employee stock option plan |  | 3,500,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_AwardTypeAxis = gpi_EmployeeStockPurchasePlanMember |  |
Employee stock purchase price in percentage of fair market value |  | 85.00%gpi_EmployeeStockPurchasePlanPurchasePricePercentageOfFairMarketValue / us-gaap_AwardTypeAxis = gpi_EmployeeStockPurchasePlanMember |  |
Shares remaining in reserve for future issuance |  | 516,710 gpi_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForFutureIssuance / us-gaap_AwardTypeAxis = gpi_EmployeeStockPurchasePlanMember |  |
Shares issued to employee under purchase plan | 106,782 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_AwardTypeAxis = gpi_EmployeeStockPurchasePlanMember | 103,254 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_AwardTypeAxis = gpi_EmployeeStockPurchasePlanMember | 104,295 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_AwardTypeAxis = gpi_EmployeeStockPurchasePlanMember |
Weighted average fair value of employee stock purchase rights issued | $ 11.90 gpi_WeightedAverageFairValueOfEmployeeStockPurchaseRightsIssued / us-gaap_AwardTypeAxis = gpi_EmployeeStockPurchasePlanMember | $ 15.15 gpi_WeightedAverageFairValueOfEmployeeStockPurchaseRightsIssued / us-gaap_AwardTypeAxis = gpi_EmployeeStockPurchasePlanMember | $ 14.37 gpi_WeightedAverageFairValueOfEmployeeStockPurchaseRightsIssued / us-gaap_AwardTypeAxis = gpi_EmployeeStockPurchasePlanMember |
Restricted Stock [Member] |  |  |  |
Stock-based Compensation Plans (Textual) [Abstract] |  |  |  |
Fair value of shares vested | $ 8,900 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember | $ 12,100 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember | $ 9,800 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember |
Restricted Stock [Member] | Maximum [Member] |  |  |  |
Stock-based Compensation Plans (Textual) [Abstract] |  |  |  |
Range of forfeiture provisions |  | 5 years |  |

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- Definition Employee service share based compensation tax benefit realized from exercise of stock options and vesting of restricted shares. + References No definition available. + Details
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- Definition Maximum number of shares available for issuance under the plan. + References No definition available. + Details
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- Definition The total cash inflow associated with the amount received from employees to purchase company stock, including stock option exercises. + References No definition available. + Details
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- Definition Shares remaining in reserve for future issuance. + References No definition available. + Details
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- Definition Weighted average fair value of employee stock purchase rights issued. + References No definition available. + Details
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- Definition Unrecognized cost of unvested share-based compensation awards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Amount of cash inflow from realized tax benefit related to deductible compensation cost reported on the entity's tax return for equity instruments in excess of the compensation cost for those instruments recognized for financial reporting purposes. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 20 -Section 55 -Paragraph 24 -URI http://asc.fasb.org/extlink&oid=32706628&loc=d3e11374-113907 + Details
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- Definition The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
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- Definition Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Date the equity-based award expires, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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