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Operating Leases - Additional Information (Detail) - USD ($) $ in Millions | Jun. 30, 2021 | Dec. 31, 2020
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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
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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