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Fair Value Measurements - Schedule of Reconciliation of Contingent Consideration Measured at Fair Value on Recurring Basis (Parenthetical) (Detail) $ in Thousands | 3 Months Ended
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Mar. 31, 2020 USD ($)
Fair Value Disclosures [Abstract] |
Reclassification to deferred consideration | $ 1,401

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- Definition Reclassification to deferred consideration. + References No definition available. + Details
Name: | mmi_ReclassificationToDeferredConsideration
Namespace Prefix: | mmi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_FairValueDisclosuresAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration