Discontinued Operations - Schedule of Operating and Investing Items Related to Discontinued Operations (Detail) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2023	Jun. 30, 2022
Operating activities of discontinued operations:
Depreciation and amortization	$ 1,457	$ 8,864
Investing activities of discontinued operations:
Purchases of rental equipment	(128,088)	(94,820)
Purchases of property, plant and equipment	(11,229)	(6,594)
Adler Tank Rentals, LLC
Operating activities of discontinued operations:
Depreciation and amortization	1,457	8,864
Gain on sale of used rental equipment	(111)	(338)
Investing activities of discontinued operations:
Proceeds from sales of used rental equipment	269	1,258
Purchases of rental equipment	(25)	(1,410)
Purchases of property, plant and equipment	$ (40)	$ (83)

Disposal group including discontinued operations net cash provided by used in operating activities.

No definition available.

Name:	mgrc_DisposalGroupIncludingDiscontinuedOperationsNetCashProvidedByUsedInOperatingActivitiesAbstract
Namespace Prefix:	mgrc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of deprecation and amortization expense attributable to property, plant and equipment and intangible assets of discontinued operations.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482740/230-10-45-28

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 205
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5B
-Subparagraph (c)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483499/205-20-50-5B

+ Details

Name:	us-gaap_DepreciationAndAmortizationDiscontinuedOperations
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of gain (loss) on sale or disposal of assets, including but not limited to property plant and equipment, intangible assets and equity in securities of subsidiaries or equity method investee.

+ References

+ Details

Name:	us-gaap_GainLossOnDispositionOfAssets1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_NoncashInvestingAndFinancingItemsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash outflow for payments to acquire rented equipment which is recorded as an asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 13
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482740/230-10-45-13

+ Details

Name:	us-gaap_PaymentsToAcquireEquipmentOnLease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash outflow associated with the acquisition of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale; includes cash outflows to pay for construction of self-constructed assets.

+ References

+ Details

Name:	us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash inflow from the sale of a lease held for investment, which may be a lease of real estate, equipment or other fixed assets for a specified time in exchange for payment, usually in the form of rent.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482740/230-10-45-12

+ Details

Name:	us-gaap_ProceedsFromLeasesHeldForInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=mgrc_AdlerTankRentalsLlcMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: