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Subsequent Events - Additional Information (Detail) - Subsequent Event [Member] $ in Millions	1 Months Ended
	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%

X

- 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

X

- 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

X

- 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: