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v3.21.1

Selected Balance Sheet Data - Schedule of Other Assets (Parenthetical) (Detail) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2021 | Mar. 31, 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