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Summary of Significant Accounting Policies - Estimated Useful Lives and Residual Values of Company's Rental Equipment (Detail) | 12 Months Ended
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Dec. 31, 2020
Relocatable modular buildings [Member] |
Property, Plant and Equipment [Line Items] |
Property subject to or available for operating lease useful life | 18 years
Percentage residual value property subject to or available for operating lease | 50.00%
Relocatable modular accessories [Member] |
Property, Plant and Equipment [Line Items] |
Percentage residual value property subject to or available for operating lease | 0.00%
Blast resistant modules [Member] |
Property, Plant and Equipment [Line Items] |
Property subject to or available for operating lease useful life | 20 years
Percentage residual value property subject to or available for operating lease | 0.00%
Portable storage containers [Member] |
Property, Plant and Equipment [Line Items] |
Property subject to or available for operating lease useful life | 25 years
Percentage residual value property subject to or available for operating lease | 62.50%
Electronic test equipment and accessories [Member] |
Property, Plant and Equipment [Line Items] |
Percentage residual value property subject to or available for operating lease | 0.00%
Liquid and solid containment tanks and boxes and accessories [Member] |
Property, Plant and Equipment [Line Items] |
Percentage residual value property subject to or available for operating lease | 0.00%
Minimum [Member] | Relocatable modular accessories [Member] |
Property, Plant and Equipment [Line Items] |
Property subject to or available for operating lease useful life | 3 years
Minimum [Member] | Electronic test equipment and accessories [Member] |
Property, Plant and Equipment [Line Items] |
Property subject to or available for operating lease useful life | 1 year
Minimum [Member] | Liquid and solid containment tanks and boxes and accessories [Member] |
Property, Plant and Equipment [Line Items] |
Property subject to or available for operating lease useful life | 3 years
Maximum [Member] | Relocatable modular accessories [Member] |
Property, Plant and Equipment [Line Items] |
Property subject to or available for operating lease useful life | 18 years
Maximum [Member] | Electronic test equipment and accessories [Member] |
Property, Plant and Equipment [Line Items] |
Property subject to or available for operating lease useful life | 8 years
Maximum [Member] | Liquid and solid containment tanks and boxes and accessories [Member] |
Property, Plant and Equipment [Line Items] |
Property subject to or available for operating lease useful life | 20 years

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- Definition Percentage residual value property subject to or available for operating lease. + References No definition available. + Details
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Namespace Prefix: | mgrc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Property subject to or available for operating lease useful life. + References No definition available. + Details
Name: | mgrc_PropertySubjectToOrAvailableForOperatingLeaseUsefulLife
Namespace Prefix: | mgrc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_PropertyPlantAndEquipmentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=mgrc_RelocatableModularBuildingsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=mgrc_RelocatableModularAccessoriesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=mgrc_BlastResistantModulesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=mgrc_PortableStorageContainersMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=mgrc_ElectronicTestEquipmentAndAccessoriesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=mgrc_LiquidAndSolidContainmentTanksAndAccessoriesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |