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Benefit Plans - Additional Information (Detail) (USD $) | 12 Months Ended |  |
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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 |  |  | 1,500,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized | 815,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized |  |
Share-based compensation expense | $ 3,854,000 us-gaap_ShareBasedCompensation | $ 3,680,000 us-gaap_ShareBasedCompensation | $ 3,840,000 us-gaap_ShareBasedCompensation |  |  |
Tax benefit related to share-based compensation expense | 1,500,000 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense | 1,400,000 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense | 1,500,000 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense |  |  |
Capitalized share-based compensation expense | 0 mgrc_CapitalizedCompensationCostsRelatedToShareBasedAwards | 0 mgrc_CapitalizedCompensationCostsRelatedToShareBasedAwards | 0 mgrc_CapitalizedCompensationCostsRelatedToShareBasedAwards |  |  |
Amount of reduction in net income due to share-based compensation expense | 2,300,000 mgrc_ReductionInNetIncomeAvailableToCommonShareholders | 2,200,000 mgrc_ReductionInNetIncomeAvailableToCommonShareholders | 2,400,000 mgrc_ReductionInNetIncomeAvailableToCommonShareholders |  |  |
Amount of reduction in diluted earning per share to share-based compensation expense | $ 0.09 mgrc_ReductionInEarningsPerShareDiluted | $ 0.09 mgrc_ReductionInEarningsPerShareDiluted | $ 0.09 mgrc_ReductionInEarningsPerShareDiluted |  |  |
Stock option plans remain outstanding | 1,343,760 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber | 1,772,062 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber | 2,838,553 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber |  | 3,086,019 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber |
Aggregate intrinsic value of options exercised and sold | 9,300,000 us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1 |  |  |  |  |
Weighted average grant date fair value | $ 9.17 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue | $ 9.87 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue | $ 10.28 us-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 |  |  |  |  |  | 1,875,000 us-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 | 16,400,000 mgrc_EmployeeStockOwnershipPlanEsopCashContributionsToEsopAssetsMerged / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = mgrc_EmployeeStockOwnershipPlanMember |  |  |  |  |
Dividends deducted | 300,000 us-gaap_EmployeeStockOwnershipPlanESOPDividendsPaidToESOP / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = mgrc_EmployeeStockOwnershipPlanMember |  |  |  |  |
Tax benefit from dividends | 100,000 us-gaap_EmployeeStockOwnershipPlanESOPTaxBenefitOfDividendsPaidToPlan / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = mgrc_EmployeeStockOwnershipPlanMember |  |  |  |  |
Shares outstanding, employee stock ownership plan | 328,400 us-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 | 1,205,444 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember |  |  |  |  |
Stock options granted | 6,821,000 mgrc_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsCumulativeNumber / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember |  |  |  |  |
Stock options, exercise prices lower | $ 3.47 mgrc_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsCumulativeNumberWeightedAverageExercisePriceRangeLower / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember |  |  |  |  |
Stock options, exercise prices upper | $ 38.89 mgrc_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsCumulativeNumberWeightedAverageExercisePriceRangeUpper / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember |  |  |  |  |
Purchase of shares | 4,583,868 mgrc_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedTillDate / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember |  |  |  |  |
Stock option shares terminated | 887,872 mgrc_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsTillDate / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember |  |  |  |  |
Stock option plans remain outstanding | 1,343,760 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember |  |  |  |  |
Stock options vesting period | 5 years |  |  |  |  |
Options issued to non-employee advisors | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember |  |  |  |  |
Stock options outstanding, exercise prices lower | $ 15.62 mgrc_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingExercisePriceRangeLowerRangeLimit / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember |  |  |  |  |
Stock options outstanding, exercise prices upper | $ 38.89 mgrc_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingExercisePriceRangeUpperRangeLimit / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember |  |  |  |  |
Aggregate intrinsic value of options exercised and sold | 8,400,000 us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1 / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember | 11,500,000 us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1 / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember | 3,900,000 us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1 / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember |  |  |
Total unrecognized compensation cost | 3,500,000 us-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 | 137,036 mgrc_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 | 18,340 mgrc_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExpectedToVest / us-gaap_AwardTypeAxis = mgrc_ServiceBasedRestrictedStockUnitsMember |  |  |  |  |
Number of forfeitures expected | 0 us-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 | 7,540 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember | 4,300 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember |  |  |  |
Stock-based compensation expense for restricted stock | 2,300,000 us-gaap_RestrictedStockExpense / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember | 2,000,000 us-gaap_RestrictedStockExpense / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember | 1,800,000 us-gaap_RestrictedStockExpense / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember |  |  |
Total unrecognized compensation expense net of forfeitures | $ 3,200,000 us-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 | 2 mgrc_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantPeriodIncreaseDecrease / us-gaap_PlanNameAxis = mgrc_TwoThousandAndSevenPlanMember |  |  |  |  |

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- Definition Employee stock ownership plan ESOP cash contributions to ESOP assets merged. + References No definition available. + Details
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- Definition Employee stock ownership plan ESOP shares in ESOP as percentage of outstanding stock. + References No definition available. + Details
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- Definition Share based compensation arrangement by share based payment award shares issued till date. + References No definition available. + Details
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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation. + References No definition available. + Details
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- Definition Unrecognized cost of unvested options awarded to employees as compensation. + References No definition available. + Details
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- Definition The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Dividends paid on stock held by an employee stock ownership plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 40 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6418621&loc=d3e17540-113929 + Details
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- Definition Sum of the allocated, committed-to-be-released and suspense shares of the entity held by the plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 40 -Section 50 -Paragraph 1 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6418621&loc=d3e17540-113929 + Details
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- Definition The tax benefit resulting from dividends paid on preferred stock held by an employee stock option plan, when that stock is not a common stock equivalent. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 40 -Section 50 -Paragraph 1 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=6418621&loc=d3e17540-113929 + Details
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- Definition The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Percentage of vesting of share-based compensation awards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Number of additional shares authorized for issuance under an established share-based compensation plan. + References No definition available. + Details
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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Number of options outstanding, including both vested and non-vested options. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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