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Notes Payable - Schedule of Information Related to Lines of Credit (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Debt Disclosure [Abstract]    
Maximum amount outstanding $ 208,894us-gaap_LineOfCreditFacilityMaximumAmountOutstandingDuringPeriod $ 202,000us-gaap_LineOfCreditFacilityMaximumAmountOutstandingDuringPeriod
Average amount outstanding $ 191,248us-gaap_LineOfCreditFacilityAverageOutstandingAmount $ 187,644us-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