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Benefit Plans - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2009
Dec. 31, 2011
Jun. 06, 2007
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Number of additional shares authorized     1,500,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized 815,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized    
Share-based compensation expense $ 3,854,000us-gaap_ShareBasedCompensation $ 3,680,000us-gaap_ShareBasedCompensation $ 3,840,000us-gaap_ShareBasedCompensation      
Tax benefit related to share-based compensation expense 1,500,000us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense 1,400,000us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense 1,500,000us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense      
Capitalized share-based compensation expense 0mgrc_CapitalizedCompensationCostsRelatedToShareBasedAwards 0mgrc_CapitalizedCompensationCostsRelatedToShareBasedAwards 0mgrc_CapitalizedCompensationCostsRelatedToShareBasedAwards      
Amount of reduction in net income due to share-based compensation expense 2,300,000mgrc_ReductionInNetIncomeAvailableToCommonShareholders 2,200,000mgrc_ReductionInNetIncomeAvailableToCommonShareholders 2,400,000mgrc_ReductionInNetIncomeAvailableToCommonShareholders      
Amount of reduction in diluted earning per share to share-based compensation expense $ 0.09mgrc_ReductionInEarningsPerShareDiluted $ 0.09mgrc_ReductionInEarningsPerShareDiluted $ 0.09mgrc_ReductionInEarningsPerShareDiluted      
Stock option plans remain outstanding 1,343,760us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber 1,772,062us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber 2,838,553us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber   3,086,019us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber  
Aggregate intrinsic value of options exercised and sold 9,300,000us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1          
Weighted average grant date fair value $ 9.17us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue $ 9.87us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue $ 10.28us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue      
Common Stock [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Shares of common stock remained available for grants of awards           1,875,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
Minimum rate of exercise price granted 100.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		          
Employee Stock Ownership and 401(k) Plans [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Plan merged assets 16,400,000mgrc_EmployeeStockOwnershipPlanEsopCashContributionsToEsopAssetsMerged
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= mgrc_EmployeeStockOwnershipPlanMember		          
Dividends deducted 300,000us-gaap_EmployeeStockOwnershipPlanESOPDividendsPaidToESOP
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= mgrc_EmployeeStockOwnershipPlanMember		          
Tax benefit from dividends 100,000us-gaap_EmployeeStockOwnershipPlanESOPTaxBenefitOfDividendsPaidToPlan
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= mgrc_EmployeeStockOwnershipPlanMember		          
Shares outstanding, employee stock ownership plan 328,400us-gaap_EmployeeStockOwnershipPlanESOPSharesInESOP
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= mgrc_EmployeeStockOwnershipPlanMember		          
Percentage of total common shares outstanding 2.00%mgrc_EmployeeStockOwnershipPlanEsopSharesInEsopAsPercentageOfOutstandingStock
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= mgrc_EmployeeStockOwnershipPlanMember		          
Stock Options [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Number of additional shares authorized 1,205,444us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		          
Stock options granted 6,821,000mgrc_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsCumulativeNumber
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		          
Stock options, exercise prices lower $ 3.47mgrc_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsCumulativeNumberWeightedAverageExercisePriceRangeLower
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		          
Stock options, exercise prices upper $ 38.89mgrc_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsCumulativeNumberWeightedAverageExercisePriceRangeUpper
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		          
Purchase of shares 4,583,868mgrc_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedTillDate
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		          
Stock option shares terminated 887,872mgrc_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsTillDate
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		          
Stock option plans remain outstanding 1,343,760us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		          
Stock options vesting period 5 years          
Options issued to non-employee advisors 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		          
Stock options outstanding, exercise prices lower $ 15.62mgrc_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingExercisePriceRangeLowerRangeLimit
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		          
Stock options outstanding, exercise prices upper $ 38.89mgrc_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingExercisePriceRangeUpperRangeLimit
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		          
Aggregate intrinsic value of options exercised and sold 8,400,000us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		 11,500,000us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		 3,900,000us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		      
Total unrecognized compensation cost 3,500,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		          
Expected recognition period of unrecognized compensation expense 2 years          
Performance-based RSUs [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Stock options vesting period 5 years          
Percentage of vesting annually at anniversary 20.00%mgrc_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingRightsPercentageVestedOnAnniversary
/ us-gaap_AwardTypeAxis
= mgrc_PerformanceBasedRestrictedStockUnitsMember		          
Number of RSU's expected to vest 137,036mgrc_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExpectedToVest
/ us-gaap_AwardTypeAxis
= mgrc_PerformanceBasedRestrictedStockUnitsMember		          
Performance-based RSUs [Member] | Vesting after three years [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Percentage of vesting RSUs 60.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
/ us-gaap_AwardTypeAxis
= mgrc_PerformanceBasedRestrictedStockUnitsMember
/ us-gaap_VestingAxis
= mgrc_VestingPeriodThreeYearsMember		          
Service-based RSUs [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Number of RSU's expected to vest 18,340mgrc_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExpectedToVest
/ us-gaap_AwardTypeAxis
= mgrc_ServiceBasedRestrictedStockUnitsMember		          
Number of forfeitures expected 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ us-gaap_AwardTypeAxis
= mgrc_ServiceBasedRestrictedStockUnitsMember		          
Restricted Stock Units [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Expected recognition period of unrecognized compensation expense 2 years 6 months          
Number of forfeitures expected 7,540us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember		 4,300us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember		        
Stock-based compensation expense for restricted stock 2,300,000us-gaap_RestrictedStockExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember		 2,000,000us-gaap_RestrictedStockExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember		 1,800,000us-gaap_RestrictedStockExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember		      
Total unrecognized compensation expense net of forfeitures $ 3,200,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember		          
Minimum [Member] | Stock Options [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Options Outstanding, Weighted-Average Remaining Contractual Life (Years) 7 years          
Minimum [Member] | Service-based RSUs [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Stock options vesting period 12 months          
Maximum [Member] | Stock Options [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Options Outstanding, Weighted-Average Remaining Contractual Life (Years) 10 years          
Maximum [Member] | Service-based RSUs [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Stock options vesting period 14 months          
2007 Plan [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Reduction in common stock available for grant 2mgrc_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantPeriodIncreaseDecrease
/ us-gaap_PlanNameAxis
= mgrc_TwoThousandAndSevenPlanMember