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Earnings Per Share (Details) (USD $)
In Thousands, except Per Share data, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Basic and Diluted Earnings per share                      
Weighted average basic common shares outstanding                 23,380us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 23,096us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 21,620us-gaap_WeightedAverageNumberOfSharesOutstandingBasic
Dilutive effect of contingently convertible notes and warrants                 1,499us-gaap_IncrementalCommonSharesAttributableToConversionOfDebtSecurities 2,213us-gaap_IncrementalCommonSharesAttributableToConversionOfDebtSecurities 1,058us-gaap_IncrementalCommonSharesAttributableToConversionOfDebtSecurities
Dilutive effect of stock options, net of assumed repurchase of treasury stock                 0gpi_DilutiveEffectOfStockOptionsNetOfAssumedRepurchaseOfTreasuryStock 0gpi_DilutiveEffectOfStockOptionsNetOfAssumedRepurchaseOfTreasuryStock 4gpi_DilutiveEffectOfStockOptionsNetOfAssumedRepurchaseOfTreasuryStock
Dilutive effect of employee stock purchases, net of assumed repurchase of treasury stock                 6gpi_DilutiveEffectOfRestrictedStockAndEmployeeStockPurchasesNetOfAssumedRepurchaseOfTreasuryStock 5gpi_DilutiveEffectOfRestrictedStockAndEmployeeStockPurchasesNetOfAssumedRepurchaseOfTreasuryStock 6gpi_DilutiveEffectOfRestrictedStockAndEmployeeStockPurchasesNetOfAssumedRepurchaseOfTreasuryStock
Weighted average dilutive common shares outstanding                 24,885us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 25,314us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 22,688us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding
Basic:                      
Net income                 $ 93,004us-gaap_NetIncomeLoss $ 113,992us-gaap_NetIncomeLoss $ 100,209us-gaap_NetIncomeLoss
Less: Earning allocated to participating securities                 3,643gpi_EarningAllocatedToParticipatingSecuritiesBasic 4,963gpi_EarningAllocatedToParticipatingSecuritiesBasic 5,269gpi_EarningAllocatedToParticipatingSecuritiesBasic
Earnings available to basic common shares                 89,361us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic 109,029us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic 94,940us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
Basic earnings per common share $ 0.77us-gaap_EarningsPerShareBasic $ 1.07us-gaap_EarningsPerShareBasic $ 0.70us-gaap_EarningsPerShareBasic $ 1.29us-gaap_EarningsPerShareBasic $ 0.89us-gaap_EarningsPerShareBasic $ 1.34us-gaap_EarningsPerShareBasic $ 1.53us-gaap_EarningsPerShareBasic $ 0.95us-gaap_EarningsPerShareBasic $ 3.82us-gaap_EarningsPerShareBasic $ 4.72us-gaap_EarningsPerShareBasic $ 4.39us-gaap_EarningsPerShareBasic
Diluted:                      
Net income                 93,004us-gaap_NetIncomeLoss 113,992us-gaap_NetIncomeLoss 100,209us-gaap_NetIncomeLoss
Less: Earning allocated to participating securities                 3,468gpi_EarningAllocatedToParticipatingSecuritiesDiluted 4,599gpi_EarningAllocatedToParticipatingSecuritiesDiluted 5,062gpi_EarningAllocatedToParticipatingSecuritiesDiluted
Earnings available to diluted common shares                 $ 89,536us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted $ 109,393us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted $ 95,147us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted
Diluted earnings per common share $ 0.77us-gaap_EarningsPerShareDiluted $ 1.03us-gaap_EarningsPerShareDiluted $ 0.62us-gaap_EarningsPerShareDiluted $ 1.19us-gaap_EarningsPerShareDiluted $ 0.81us-gaap_EarningsPerShareDiluted $ 1.19us-gaap_EarningsPerShareDiluted $ 1.43us-gaap_EarningsPerShareDiluted $ 0.88us-gaap_EarningsPerShareDiluted $ 3.60us-gaap_EarningsPerShareDiluted $ 4.32us-gaap_EarningsPerShareDiluted $ 4.19us-gaap_EarningsPerShareDiluted