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Notes Payable - Schedule of Information Related to Lines of Credit (Detail) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Debt Disclosure [Abstract] |  |
Maximum amount outstanding | $ 208,894 us-gaap_LineOfCreditFacilityMaximumAmountOutstandingDuringPeriod | $ 202,000 us-gaap_LineOfCreditFacilityMaximumAmountOutstandingDuringPeriod
Average amount outstanding | $ 191,248 us-gaap_LineOfCreditFacilityAverageOutstandingAmount | $ 187,644 us-gaap_LineOfCreditFacilityAverageOutstandingAmount
Weighted average interest rate, during the period | 2.41%mgrc_LineOfCreditFacilityWeightedInterestRateDuringPeriod | 2.45%mgrc_LineOfCreditFacilityWeightedInterestRateDuringPeriod
Prime interest rate, end of period | 3.25%mgrc_LineOfCreditFacilityBaseInterestRateAtPeriodEnd | 3.25%mgrc_LineOfCreditFacilityBaseInterestRateAtPeriodEnd

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- 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

X
- 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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- 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