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Notes Payable - Schedule of Information Related to Lines of Credit (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Maximum amount outstanding	$ 215,732	$ 239,820
Average amount outstanding	$ 199,499	$ 214,446
Weighted average interest rate, during the period	2.64%	2.19%
Prime interest rate, end of period	4.50%	3.75%

X

- Definition

Line of credit facility base interest rate at period end.

+ References

No definition available.

+ Details

Name:	mgrc_LineOfCreditFacilityBaseInterestRateAtPeriodEnd
Namespace Prefix:	mgrc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Line of credit facility weighted interest rate during period.

+ References

No definition available.

+ Details

Name:	mgrc_LineOfCreditFacilityWeightedInterestRateDuringPeriod
Namespace Prefix:	mgrc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- References

No definition available.

+ Details

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

X

- Definition

Average amount borrowed under the credit facility during the period.

+ References

No definition available.

+ Details

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

X

- Definition

Maximum amount borrowed under the credit facility at any time during the period.

+ References

No definition available.

+ Details

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