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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		$ 41,400us-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		16,012us-gaap_ShareBasedCompensation	13,899us-gaap_ShareBasedCompensation	11,931us-gaap_ShareBasedCompensation
Income tax benefit for stock based compensation arrangements	3,200us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense	4,400us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense	3,700us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
Cash received from option exercises and employee stock purchase plan purchases	5,700gpi_ProceedsFromOptionExercisesAndEmployeeStockPurchasePlanPurchases	6,000gpi_ProceedsFromOptionExercisesAndEmployeeStockPurchasePlanPurchases	5,600gpi_ProceedsFromOptionExercisesAndEmployeeStockPurchasePlanPurchases
Tax benefit realized for the tax deductions	3,000gpi_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptionsAndVestingOfRestrictedShares	1,841gpi_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptionsAndVestingOfRestrictedShares	3,000gpi_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptionsAndVestingOfRestrictedShares
Increase in financing activities and decrease in operating activities due to tax benefit relating to excess stock based compensation deductions		1,841us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities	2,993us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities	2,875us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities
2007 Long Term Incentive Plan [Member]
Stock-based Compensation Plans (Textual) [Abstract]
Maximum number of shares available for issuance under the plan		7,500,000gpi_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		1,200,000gpi_MaximumNumberOfSharesAvailableForIssuanceUnderPlan / us-gaap_AwardTypeAxis = gpi_A2014LongTermIncentivePlanMember
Expiry date for stock options grant		May 21, 2024
Shares available for granted under the incentive plan		1,711,034us-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		3,500,000us-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		516,710gpi_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForFutureIssuance / us-gaap_AwardTypeAxis = gpi_EmployeeStockPurchasePlanMember
Shares issued to employee under purchase plan	106,782us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_AwardTypeAxis = gpi_EmployeeStockPurchasePlanMember	103,254us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_AwardTypeAxis = gpi_EmployeeStockPurchasePlanMember	104,295us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_AwardTypeAxis = gpi_EmployeeStockPurchasePlanMember
Weighted average fair value of employee stock purchase rights issued	$ 11.90gpi_WeightedAverageFairValueOfEmployeeStockPurchaseRightsIssued / us-gaap_AwardTypeAxis = gpi_EmployeeStockPurchasePlanMember	$ 15.15gpi_WeightedAverageFairValueOfEmployeeStockPurchaseRightsIssued / us-gaap_AwardTypeAxis = gpi_EmployeeStockPurchasePlanMember	$ 14.37gpi_WeightedAverageFairValueOfEmployeeStockPurchaseRightsIssued / us-gaap_AwardTypeAxis = gpi_EmployeeStockPurchasePlanMember
Restricted Stock [Member]
Stock-based Compensation Plans (Textual) [Abstract]
Fair value of shares vested	$ 8,900us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	$ 12,100us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	$ 9,800us-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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Employee service share based compensation tax benefit realized from exercise of stock options and vesting of restricted shares.

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Employee common stock purchase price in percentage of fair market value.

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Maximum number of shares available for issuance under the plan.

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The total cash inflow associated with the amount received from employees to purchase company stock, including stock option exercises.

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Range of forfeiture provisions.

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Shares remaining in reserve for future issuance.

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Weighted average fair value of employee stock purchase rights issued.

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Unrecognized cost of unvested share-based compensation awards.

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-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

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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.

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The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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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.

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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.

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-Name Accounting Standards Codification

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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.

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Date the equity-based award expires, in CCYY-MM-DD format.

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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).

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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.

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Number of shares issued during the period as a result of an employee stock purchase plan.

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

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