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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
	Mar. 31, 2020 USD ($)
Fair Value Disclosures [Abstract]
Reclassification to deferred consideration	$ 1,401

X

- 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

X

- References

No definition available.

+ Details

Name:	us-gaap_FairValueDisclosuresAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration