Recent Accounting Pronouncements - Additional Information (Detail) - ASU 2016-09 [Member] - USD ($)	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2016
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Excess tax benefits related to share-based payments, reduction to provision for income taxes	$ 360,000	$ 494,000
Decreases to both net cash provided by operations and net cash used in financing activities			$ 993,000

New accounting pronouncement or change in accounting principle, decreases to both net cash provided by operations and net cash used in financing activities.

No definition available.

Name:	mgrc_NewAccountingPronouncementOrChangeInAccountingPrincipleDecreasesToBothNetCashProvidedByOperationsAndNetCashUsedInFinancingActivities
Namespace Prefix:	mgrc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration