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Earnings Per Share (Details Textual)
Mar. 31, 2015
2.25% Convertible Notes [Member]  
Earnings Per Share (Textual) [Abstract]  
Debt interest rate 2.25%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_LongtermDebtTypeAxis
= gpi_A2.25ConvertibleNotesMember
2.25% Warrants [Member]  
Earnings Per Share (Textual) [Abstract]  
Debt interest rate 2.25%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_LongtermDebtTypeAxis
= gpi_TwoPointTwoFivePercentWarrantsMember
3.00% Notes [Member]  
Earnings Per Share (Textual) [Abstract]  
Debt interest rate 3.00%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_LongtermDebtTypeAxis
= gpi_ThreePointZeroZeroPercentNotesMember
3.00% Warrants [Member]  
Earnings Per Share (Textual) [Abstract]  
Debt interest rate 3.00%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_WarrantMember