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Commitments and Contingencies - Additional Information (Detail) - USD ($) | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract] |  |  |
Facility rent expense | $ 3,900,000 | $ 3,500,000 | $ 3,500,000
Health plan annual stop-loss insurance per claim | 200,000 |  |
Workers compensation insurance with no stop-loss for prior claim years | 350,000 |  |
Accruals for health and workers' compensation plans | $ 2,800,000 | $ 3,000,000.0 |

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- Definition Health and workers compensation plans liability. + References No definition available. + Details
Name: | mgrc_HealthAndWorkersCompensationPlansLiability
Namespace Prefix: | mgrc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Health plan annual stop-loss insurance per claim. + References No definition available. + Details
Name: | mgrc_HealthPlanAnnualStopLossInsurancePerClaim
Namespace Prefix: | mgrc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Workers compensation insurance with no stop-loss for prior claim years. + References No definition available. + Details
Name: | mgrc_WorkersCompensationInsuranceWithNoStopLossForPriorClaimYears
Namespace Prefix: | mgrc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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- Definition Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41499-112717 + Details
Name: | us-gaap_OperatingLeasesRentExpenseNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration