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Income Taxes - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Operating Loss Carryforwards [Line Items]      
Excess tax benefit obtained by company $ 1,800,000us-gaap_DeferredTaxExpenseFromStockOptionsExercised $ 1,300,000us-gaap_DeferredTaxExpenseFromStockOptionsExercised $ 1,000,000us-gaap_DeferredTaxExpenseFromStockOptionsExercised
Undistributed earnings of foreign subsidiaries 1,600,000us-gaap_UndistributedEarningsOfForeignSubsidiaries 1,200,000us-gaap_UndistributedEarningsOfForeignSubsidiaries  
Likelihood percentage of being realized upon ultimate settlement with the relevant tax authority 50.00%mgrc_TaxBenefitLikelihoodPercentage    
Unrecognized tax benefits 0us-gaap_UnrecognizedTaxBenefits 0us-gaap_UnrecognizedTaxBenefits  
Changes in unrecognized benefits 0us-gaap_UnrecognizedTaxBenefitsPeriodIncreaseDecrease 0us-gaap_UnrecognizedTaxBenefitsPeriodIncreaseDecrease 0us-gaap_UnrecognizedTaxBenefitsPeriodIncreaseDecrease
Foreign [Member]      
Operating Loss Carryforwards [Line Items]      
Company's foreign net operating losses for tax purposes 1,000,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_ForeignCountryMember		    
Foreign net operating loss carry forwards, expiration description Begin to expire in 2022    
State and Local Jurisdiction [Member]      
Operating Loss Carryforwards [Line Items]      
State and foreign tax credit carry forwards   $ 2,600,000us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember