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Operating Leases - Additional Information (Detail) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Leases [Abstract]
Operating lease right-of-use assets	$ 138.8	$ 126.9
Operating lease right-of-use assets, accumulated amortization	$ 52.4	$ 42.9

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- Definition

Accumulated Amortization Of Right Of Use Lease Assets

+ References

No definition available.

+ Details

Name:	mmi_AccumulatedAmortizationOfRightOfUseLeaseAssets
Namespace Prefix:	mmi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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- Definition

Operating Lease Right Of Use Asset Gross.

+ References

No definition available.

+ Details

Name:	mmi_OperatingLeaseRightOfUseAssetGross
Namespace Prefix:	mmi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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- References

No definition available.

+ Details

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