Stock-Based Compensation Plans (Details Textual) (USD $)	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Additional Stock-based Compensation Plans (Textual) [Abstract]
Cash received from option exercises and employee stock purchase plan purchases	$ 1,900,000	$ 1,600,000
Tax benefit realized for the tax deductions	1,000,000	700,000
Share-based Compensation	$ 4,888,000us-gaap_ShareBasedCompensation	$ 3,660,000us-gaap_ShareBasedCompensation
2014 Long Term Incentive Plan [Member]
Stock-based Compensation Plans (Textual) [Abstract]
Expiry date for stock options grant	May 21, 2024
Shares available for granted under the incentive plan	1,529,795
Employee Stock Purchase Plan [Member]
Stock-based Compensation Plans (Textual) [Abstract]
Expiry date for stock options grant	Mar. 06, 2016
Shares authorized for issuance	3,500,000
Employee stock purchase price in percentage of fair market value	85.00%
Shares remaining in reserve for future issuance	490,430
Shares issued to employee under purchase plan	26,280	28,451
Weighted average fair value of employee stock purchase rights issued	$ 18.72	$ 14.85
Maximum [Member]
Stock-based Compensation Plans (Textual) [Abstract]
Range Of Forfeiture Provisions	5 years

Name:	gpi_AdditionalStockBasedCompensationPlansTextualAbstract
Namespace Prefix:	gpi_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Employee service share based compensation tax benefit realized from exercise of stock options and vesting of restricted shares.

+ References

No definition available.

+ Details

Name:	gpi_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptionsAndVestingOfRestrictedShares
Namespace Prefix:	gpi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

Name:	gpi_EmployeeStockPurchasePlanPurchasePricePercentageOfFairMarketValue
Namespace Prefix:	gpi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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

Name:	gpi_ProceedsFromOptionExercisesAndEmployeeStockPurchasePlanPurchases
Namespace Prefix:	gpi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Range of forfeiture provisions.

+ References

No definition available.

+ Details

Name:	gpi_RangeOfForfeitureProvisions
Namespace Prefix:	gpi_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Shares remaining in reserve for future issuance.

+ References

No definition available.

+ Details

Name:	gpi_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForFutureIssuance
Namespace Prefix:	gpi_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	gpi_StockBasedCompensationPlansTextualAbstract
Namespace Prefix:	gpi_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted average fair value of employee stock purchase rights issued.

+ References

No definition available.

+ Details

Name:	gpi_WeightedAverageFairValueOfEmployeeStockPurchaseRightsIssued
Namespace Prefix:	gpi_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- 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

Name:	us-gaap_ShareBasedCompensation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardExpirationDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- 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

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- 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

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.3-04)

-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 29, 30

-Article 5

+ Details

Name:	us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_AwardTypeAxis=gpi_A2014LongTermIncentivePlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=gpi_EmployeeStockPurchasePlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: