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Summary of Significant Accounting Policies and Estimates - Self-Insured Medical, Property and Casualty Reserves (Details) | 12 Months Ended
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Dec. 31, 2018 USD ($)
Insurance Reserves [Line Items] |
Maximum payout | $ 0
Exposure for insurance coverage per occurrence | 1,000,000
Estimated liability on worker's compensation and general liability claims | 19,300,000
Maximum |
Insurance Reserves [Line Items] |
Auto physical damage insurance coverage deductible per occurrence | 10,000,000
Maximum aggregate deductible | $ 30,000,000

X
- Definition Aggregate deductible + References No definition available. + Details
Name: | gpi_AggregateDeductible
Namespace Prefix: | gpi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition General Liability And Workers' Compensation Liability + References No definition available. + Details
Name: | gpi_GeneralLiabilityAndWorkersCompensationLiability
Namespace Prefix: | gpi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition [Line Items] for Insurance Reserves [Table] + References No definition available. + Details
Name: | gpi_InsuranceReservesLineItems
Namespace Prefix: | gpi_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Limited exposure in no stop loss plan per occurrence. + References No definition available. + Details
Name: | gpi_LimitedExposureInNoStopLossPlanPerOccurrence
Namespace Prefix: | gpi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Maximum payout under first layer + References No definition available. + Details
Name: | gpi_Maximumpayoutunderfirstlayer
Namespace Prefix: | gpi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Layer of coverage on the Company's auto physical damage insurance + References No definition available. + Details
Name: | gpi_PerOccurrenceCompanyDeductible
Namespace Prefix: | gpi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |