Commitments and Contingencies (Details Textual) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015
Commitments and Contingencies (Textual) [Abstract]
Rental payment obligations during the remaining terms of these leases	$ 2.6gpi_SecondaryGuarantorLeaseObligations

Total lease obligation for the period when the Company is the guarantor on a lease, sublets and becomes the secondary guarantor.

No definition available.

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