Condensed Consolidated Financial Information Condensed Consolidated Cash Flow (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Net Cash Provided by (Used in) Operating Activities	$ 198,288	$ 52,372	$ (75,322)
Payments to Acquire Businesses, Net of Cash Acquired	(336,551)	(269,860)	(177,956)
Proceeds From Disposition Of Franchise Property And Equipment	144,597	102,186	39,197
Payments to Acquire Property, Plant, and Equipment	(150,392)	(102,858)	(88,491)
Payments for (Proceeds from) Other Investing Activities	(4,705)	1,878	2,792
Net Cash Provided by (Used in) Investing Activities	(347,051)	(268,654)	(224,458)
Borrowings On Credit Facility Floorplan Line	7,832,014	6,379,328	5,700,108
Repayments On Credit Facility Floorplan Line	(7,802,719)	(6,153,677)	(5,453,148)
Borrowings on Credit Facility Acquisition Line	389,368	60,000	0
Repayments on Credit Facility Acquisition Line	(379,681)	0	0
Borrowings On Real Estate Credit Facility	200	19,640	18,080
Principal Payments On Real Estate Credit Facility	(9,917)	(8,597)	(2,406)
Proceeds from Issuance of Unsecured Debt	539,600	0	0
Payments of Debt Issuance Costs	(1,881)us-gaap_PaymentsOfDebtIssuanceCosts	0us-gaap_PaymentsOfDebtIssuanceCosts	0us-gaap_PaymentsOfDebtIssuanceCosts
Repayments of 3.00% Convertible Notes	(260,074)gpi_Repaymentsof3.00ConvertibleNotes	0gpi_Repaymentsof3.00ConvertibleNotes	0gpi_Repaymentsof3.00ConvertibleNotes
Proceeds from call options and warrants related to 3% Notes	32,697	0	0
Repayments of 2.25% Convertible Notes	(182,756)gpi_Repaymentsof2.25ConvertibleNotes	0gpi_Repaymentsof2.25ConvertibleNotes	0gpi_Repaymentsof2.25ConvertibleNotes
Proceeds from Issuance of Other Long-term Debt	91,137	10,289	275
Repayments of Other Long-term Debt	(85,905)us-gaap_RepaymentsOfOtherLongTermDebt	(71,170)us-gaap_RepaymentsOfOtherLongTermDebt	(4,784)us-gaap_RepaymentsOfOtherLongTermDebt
Borrowings Of Long Term Debt Related To Real Estate Loans	111,979	55,345	70,685
Principal Payments Of Long Term Debt Related To Real Estate Loans	(50,033)	(36,978)	(15,197)
Proceeds from Issuance of Shares under Incentive and Share-based Compensation Plans, Including Stock Options	(321)	(1,822)	(915)
Payments for Repurchase of Common Stock	(36,802)	(3,553)	(11,317)
Tax effect from excess stock-based compensation	1,841	2,993	2,875
Payments of Ordinary Dividends, Common Stock	(17,097)us-gaap_PaymentsOfDividendsCommonStock	(15,805)us-gaap_PaymentsOfDividendsCommonStock	(13,433)us-gaap_PaymentsOfDividendsCommonStock
Borrowings (repayments) with subsidiaries	0	0	0
Change in investment in subsidiaries	0gpi_Changeininvestmentinsubsidiaries	0gpi_Changeininvestmentinsubsidiaries	0gpi_Changeininvestmentinsubsidiaries
Cash Dividends Paid to Parent Company by Consolidated Subsidiaries	0	0	0
Investment in Subsidiaries	0gpi_InvestmentinSubsidiaries	0gpi_InvestmentinSubsidiaries
Net Cash Provided by (Used in) Financing Activities	171,650	235,993	290,823
Effect of Exchange Rate on Cash and Cash Equivalents	(2,127)	(4,146)	(1,288)
Cash and Cash Equivalents, Period Increase (Decrease)	20,760	15,565	(10,245)
Cash and Cash Equivalents, at Carrying Value	40,975	20,215	4,650	14,895
Parent Company [Member]
Net Cash Provided by (Used in) Operating Activities	(327)	(3,797)	(2,851)
Payments to Acquire Businesses, Net of Cash Acquired		0	0
Proceeds From Disposition Of Franchise Property And Equipment	0	0	0
Payments to Acquire Property, Plant, and Equipment	0	0	0
Payments for (Proceeds from) Other Investing Activities	0	0	0
Net Cash Provided by (Used in) Investing Activities	0	0	0
Borrowings On Credit Facility Floorplan Line	0	0	0
Repayments On Credit Facility Floorplan Line	0	0	0
Borrowings on Credit Facility Acquisition Line	389,368	60,000	0
Repayments on Credit Facility Acquisition Line	(379,681)		0
Borrowings On Real Estate Credit Facility	0	0	0
Principal Payments On Real Estate Credit Facility	0	0	0
Proceeds from Issuance of Unsecured Debt	539,600
Payments of Debt Issuance Costs	(1,881)
Repayments of 3.00% Convertible Notes	(260,074)
Proceeds from call options and warrants related to 3% Notes	32,697
Repayments of 2.25% Convertible Notes	(182,756)
Proceeds from Issuance of Other Long-term Debt	0	0	0
Repayments of Other Long-term Debt	0	0	0
Borrowings Of Long Term Debt Related To Real Estate Loans	0	0	0
Principal Payments Of Long Term Debt Related To Real Estate Loans	0	0	0
Proceeds from Issuance of Shares under Incentive and Share-based Compensation Plans, Including Stock Options	(321)	(1,822)	(915)
Payments for Repurchase of Common Stock	(36,802)	(3,553)	(11,317)
Tax effect from excess stock-based compensation	0	0	0
Payments of Ordinary Dividends, Common Stock	(17,097)	(15,805)	(13,433)
Borrowings (repayments) with subsidiaries	78,199	50,651	145,184
Change in investment in subsidiaries	(160,925)	(85,674)	(116,668)
Cash Dividends Paid to Parent Company by Consolidated Subsidiaries	0	0	0
Investment in Subsidiaries	(1,964,442)	(1,710,185)
Net Cash Provided by (Used in) Financing Activities	327	3,797	2,851
Effect of Exchange Rate on Cash and Cash Equivalents	0	0	0
Cash and Cash Equivalents, Period Increase (Decrease)	0	0	0
Cash and Cash Equivalents, at Carrying Value	0	0	0	0
Guarantor Subsidiaries [Member]
Net Cash Provided by (Used in) Operating Activities	235,236	47,525	(78,501)
Payments to Acquire Businesses, Net of Cash Acquired	(306,364)	(131,654)	(155,299)
Proceeds From Disposition Of Franchise Property And Equipment	141,147	102,069	39,122
Payments to Acquire Property, Plant, and Equipment	(140,407)	(73,615)	(84,933)
Payments for (Proceeds from) Other Investing Activities	(4,705)	1,878	2,792
Net Cash Provided by (Used in) Investing Activities	(310,329)	(101,322)	(198,318)
Borrowings On Credit Facility Floorplan Line	7,832,014	6,379,328	5,700,108
Repayments On Credit Facility Floorplan Line	(7,802,719)	(6,153,677)	(5,453,148)
Borrowings on Credit Facility Acquisition Line	0	0	0
Repayments on Credit Facility Acquisition Line	0		0
Borrowings On Real Estate Credit Facility	200	19,640	18,080
Principal Payments On Real Estate Credit Facility	(9,917)	(8,597)	(2,406)
Proceeds from Issuance of Unsecured Debt	0
Payments of Debt Issuance Costs	0
Repayments of 3.00% Convertible Notes	0
Proceeds from call options and warrants related to 3% Notes	0
Repayments of 2.25% Convertible Notes	0
Proceeds from Issuance of Other Long-term Debt	0	0	275
Repayments of Other Long-term Debt	0	0	(3,846)
Borrowings Of Long Term Debt Related To Real Estate Loans	86,522	27,925	62,895
Principal Payments Of Long Term Debt Related To Real Estate Loans	(33,143)	(21,038)	(13,098)
Proceeds from Issuance of Shares under Incentive and Share-based Compensation Plans, Including Stock Options	0	0	0
Payments for Repurchase of Common Stock	0	0	0
Tax effect from excess stock-based compensation	1,841	2,993	2,875
Payments of Ordinary Dividends, Common Stock	0	0	0
Borrowings (repayments) with subsidiaries	(141,824)	(278,850)	(146,525)
Change in investment in subsidiaries	161,073	86,038	108,039
Cash Dividends Paid to Parent Company by Consolidated Subsidiaries	2,119	0	1,049
Investment in Subsidiaries	0	0
Net Cash Provided by (Used in) Financing Activities	96,166	53,762	274,298
Effect of Exchange Rate on Cash and Cash Equivalents	0	0	0
Cash and Cash Equivalents, Period Increase (Decrease)	21,073	(35)	(2,521)
Cash and Cash Equivalents, at Carrying Value	25,379	4,306	4,341	6,862
Non-Guarantor Subsidiaries [Member]
Net Cash Provided by (Used in) Operating Activities	(36,621)	8,644	6,030
Payments to Acquire Businesses, Net of Cash Acquired	(30,187)	(138,206)	(22,657)
Proceeds From Disposition Of Franchise Property And Equipment	3,450	117	75
Payments to Acquire Property, Plant, and Equipment	(9,985)	(29,243)	(3,558)
Payments for (Proceeds from) Other Investing Activities	0	0	0
Net Cash Provided by (Used in) Investing Activities	(36,722)	(167,332)	(26,140)
Borrowings On Credit Facility Floorplan Line	0	0	0
Repayments On Credit Facility Floorplan Line	0	0	0
Borrowings on Credit Facility Acquisition Line	0	0	0
Repayments on Credit Facility Acquisition Line	0		0
Borrowings On Real Estate Credit Facility	0	0	0
Principal Payments On Real Estate Credit Facility	0	0	0
Proceeds from Issuance of Unsecured Debt	0
Payments of Debt Issuance Costs	0
Repayments of 3.00% Convertible Notes	0
Proceeds from call options and warrants related to 3% Notes	0
Repayments of 2.25% Convertible Notes	0
Proceeds from Issuance of Other Long-term Debt	91,137	10,289	0
Repayments of Other Long-term Debt	(85,905)	(71,170)	(938)
Borrowings Of Long Term Debt Related To Real Estate Loans	25,457	27,420	7,790
Principal Payments Of Long Term Debt Related To Real Estate Loans	(16,890)	(15,940)	(2,099)
Proceeds from Issuance of Shares under Incentive and Share-based Compensation Plans, Including Stock Options	0	0	0
Payments for Repurchase of Common Stock	0	0	0
Tax effect from excess stock-based compensation	0	0	0
Payments of Ordinary Dividends, Common Stock	0	0	0
Borrowings (repayments) with subsidiaries	63,625	228,199	1,341
Change in investment in subsidiaries	(148)	(364)	8,629
Cash Dividends Paid to Parent Company by Consolidated Subsidiaries	(2,119)	0	(1,049)
Investment in Subsidiaries	0	0
Net Cash Provided by (Used in) Financing Activities	75,157	178,434	13,674
Effect of Exchange Rate on Cash and Cash Equivalents	(2,127)	(4,146)	(1,288)
Cash and Cash Equivalents, Period Increase (Decrease)	(313)	15,600	(7,724)
Cash and Cash Equivalents, at Carrying Value	15,596	15,909	309	8,033
Consolidation, Eliminations [Member]
Investment in Subsidiaries	1,964,442	1,710,185
Cash and Cash Equivalents, at Carrying Value	$ 0	$ 0

Borrowings of long term debt related to real estate loans.

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Borrowings on credit facility acquisition line.

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The cash inflow resulting from borrowings under the floorplan line of the revolving credit facility. Borrowings under the floorplan line are used to finance the purchase of specific new, used and rental vehicle inventory.

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Borrowings on real estate credit facility.

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Borrowings (repayments) with subsidiaries

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Change in investment in subsidiaries

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Investment in Subsidiaries

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The cash outflow resulting from principal payments on borrowings under real estate loans.

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The cash outflow resulting from repayments of borrowings under the real estate credit facility.

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Proceeds from call options and warrants related to 3% Notes

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Cash inflow from the sale of long-lived physical assets, dealership franchises, and related real estate, as applicable.

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Repayments of 2.25% Convertible Notes

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Repayments of 3.00% Convertible Notes

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Repayments on credit facility acquisition line.

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The cash outflow resulting from repayments of borrowings under the floorplan line of the revolving credit facility. Borrowings under the floorplan line are used to finance the purchase of specific new, used and rental vehicle inventory.

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No definition available.

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Amount of increase (decrease) in cash and cash equivalents. Cash and cash equivalents are the amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Includes effect from exchange rate changes.

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This element represents disclosure of the aggregate cash dividends paid to the entity by consolidated subsidiaries.

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Amount of increase (decrease) from the effect of exchange rate changes on cash and cash equivalent balances held in foreign currencies.

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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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Amount of cash inflow (outflow) from financing activities, including discontinued operations. Financing activity cash flows include obtaining resources from owners and providing them with a return on, and a return of, their investment; borrowing money and repaying amounts borrowed, or settling the obligation; and obtaining and paying for other resources obtained from creditors on long-term credit.

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Amount of cash inflow (outflow) from investing activities, including discontinued operations. Investing activity cash flows include making and collecting loans and acquiring and disposing of debt or equity instruments and property, plant, and equipment and other productive assets.

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Amount of cash inflow (outflow) from operating activities, including discontinued operations. Operating activity cash flows include transactions, adjustments, and changes in value not defined as investing or financing activities.

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The net cash outflow or inflow from other investing activities. This element is used when there is not a more specific and appropriate element in the taxonomy.

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The cash outflow to reacquire common stock during the period.

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-Publisher FASB

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-Glossary Financing Activities

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-Paragraph 15

-Subparagraph (a)

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The cash outflow paid to third parties in connection with debt origination, which will be amortized over the remaining maturity period of the associated long-term debt.

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Amount of cash outflow in the form of ordinary dividends to common shareholders of the parent entity.

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The cash outflow associated with the acquisition of a business, net of the cash acquired from the purchase.

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The cash outflow associated with the acquisition of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale; includes cash outflows to pay for construction of self-constructed assets.

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The cash inflow from other long-term borrowings not otherwise defined in the taxonomy (with maturities initially due beyond one year or the normal operating cycle of the entity, if longer).

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The total cash inflow associated with the amount received from holders to acquire the entity's shares under incentive and share awards, including stock option exercises. This item inherently excludes any excess tax benefit, which the entity may have realized and reported separately.

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The cash inflow from the issuance of long-term debt that is not secured by collateral. Excludes proceeds from tax exempt unsecured debt.

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The cash outflow attributable to repayments of borrowings not otherwise defined in the taxonomy (with maturities initially due after one year or beyond the operating cycle if longer).

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