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Notes Payable - Schedule of Information Related to Lines of Credit (Detail) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017
Debt Disclosure [Abstract] |  |
Maximum amount outstanding | $ 228,500 | $ 215,732
Average amount outstanding | $ 204,534 | $ 199,499
Weighted average interest rate, during the period | 3.36% | 2.64%
Prime interest rate, end of period | 5.50% | 4.50%

X
- Definition Line of credit facility base interest rate at period end. + References No definition available. + Details
Name: | mgrc_LineOfCreditFacilityBaseInterestRateAtPeriodEnd
Namespace Prefix: | mgrc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Line of credit facility weighted interest rate during period. + References No definition available. + Details
Name: | mgrc_LineOfCreditFacilityWeightedInterestRateDuringPeriod
Namespace Prefix: | mgrc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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

X
- Definition Average amount borrowed under the credit facility during the period. + References No definition available. + Details
Name: | us-gaap_LineOfCreditFacilityAverageOutstandingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Maximum amount borrowed under the credit facility at any time during the period. + References No definition available. + Details
Name: | us-gaap_LineOfCreditFacilityMaximumAmountOutstandingDuringPeriod
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration