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Selected Balance Sheet Data - Schedule of Other Assets (Parenthetical) (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Other Assets [Line Items]
Reduction of accrued bonuses and other employee related expenses in settlement of employee notes receivable	$ 10	$ 0

X

- Definition

Other Assets [Line Items]

+ References

No definition available.

+ Details

Name:	mmi_OtherAssetsLineItems
Namespace Prefix:	mmi_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Reduction of accrued bonuses and other employee related expenses in settlement of employee notes receivable.

+ References

No definition available.

+ Details

Name:	mmi_ReductionOfAccruedBonusesAndOtherEmployeeRelatedExpensesInSettlementOfEmployeeNotesReceivable
Namespace Prefix:	mmi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration