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Summary of Significant Accounting Policies and Estimates - Advertising (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Accounting Policies [Abstract] |  |  |
Advertising expense | $ 75.2 | $ 74.1 | $ 75.3
Reduction in advertising expense for advertising assistance earned related to vehicles sold | $ 14.8 | $ 15.3 | $ 16.7

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- Definition Manufacturer advertising assistance recognized as a reduction of advertising expense. + References No definition available. + Details
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Namespace Prefix: | gpi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount charged to advertising expense for the period, which are expenses incurred with the objective of increasing revenue for a specified brand, product or product line. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 720 -SubTopic 35 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6420018&loc=d3e36677-107848 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration