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v3.20.1

Subsequent Events - Additional Information (Detail) - Subsequent Event [Member] $ in Millions | 1 Months Ended
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May 11, 2020 USD ($)
Subsequent Event [Line Items] |
Total consideration for business acquisitions and other commitments | $ 17.9
COVID-19 [Member] |
Subsequent Event [Line Items] |
Reduction of employee workforce | 20.00%

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- Definition Consideration for commitments to investments sales and financing professionals and business acquisitions. + References No definition available. + Details
Name: | mmi_TotalConsiderationForBusinessAcquisitionsAndOtherCommitments
Namespace Prefix: | mmi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The number of positions eliminated during the period as a percentage of total positions eliminated during the period in connection with the restructuring plan(s). + References No definition available. + Details
Name: | us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminatedPeriodPercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_UnusualOrInfrequentItemAxis=mmi_COVIDNineteenMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |