Additional borrowings discount from face value

No definition available.

Name:	gpi_Additionalborrowingsdiscountfromfacevalue
Namespace Prefix:	gpi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Additional borrowings of long term debt

+ References

No definition available.

+ Details

Name:	gpi_Additionalborrowingsoflongtermdebt
Namespace Prefix:	gpi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amortization period of underwriters fees and debt issuance costs

+ References

No definition available.

+ Details

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

- Definition

Carrying value of Notes related discount and equity component.

+ References

No definition available.

+ Details

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

- Definition

Redemption price percentage of principal outstanding and accrued unpaid interest

+ References

No definition available.

+ Details

Name:	gpi_Redemptionpricepercentageofprincipaloutstandingandaccruedunpaidinterest
Namespace Prefix:	gpi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Redemption price percentage of principal redeemed

+ References

No definition available.

+ Details

Name:	gpi_Redemptionpricepercentageofprincipalredeemed
Namespace Prefix:	gpi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 55

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400

Reference 2: 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

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: 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

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Latest date the outstanding debt instruments are required to be repaid, in CCYY-MM-DD format.

+ References

Reference 1: 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)(2))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentMaturityDateRangeEnd1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of principal amount of debt redeemed.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentRedemptionPricePercentageOfPrincipalAmountRedeemed
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of debt issuance costs (for example, but not limited to, legal, accounting, broker, and regulatory fees).

+ References

No definition available.

+ Details

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

- Definition

The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.17)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_UnamortizedDebtIssuanceExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of deferred fees paid by borrowers and unamortized costs incurred to originate loans and leases, unamortized loan commitments and loan syndication fees, and premiums over or discounts from face amounts of loans that are being amortized into income as an adjustment to yield. Excludes amounts for loans and leases covered under loss sharing agreements.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 20

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6378536&loc=d3e10092-111533

+ Details

Name:	us-gaap_UnamortizedLoanCommitmentAndOriginationFeesAndUnamortizedDiscountsOrPremiums
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_LongtermDebtTypeAxis=gpi_A5.00SeniorNoteMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Long-Term Debt (Details 1) - USD ($) $ in Millions	6 Months Ended
Long-Term Debt (Details 1) - USD ($) $ in Millions	Jun. 30, 2015	Sep. 30, 2014	Jun. 30, 2014
Carrying value of Notes related discount and equity component
Amortization period of underwriters fees and debt issuance costs	8 years
5.00% Senior Note [Member]
Carrying value of Notes related discount and equity component
Debt Instrument, Face Amount	$ 550.0		$ 350.0
Debt Instrument, Interest Rate, Stated Percentage	5.00%
Debt Issuance Cost	$ 2.6
Additional borrowings of long term debt		$ 200.0
Additional borrowings discount from face value	1.50%
Debt Instrument, Maturity Date Range, End	Jun. 01, 2022
Debt Instrument, Redemption Price, Percentage of Principal Amount Redeemed	35.00%
Redemption price percentage of principal outstanding and accrued unpaid interest	105.00%
Redemption price percentage of principal redeemed	100.00%
Unamortized underwriter fees	$ (9.3)
Unamortized debt issuance cost	$ 2.2