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Fair Value Measurements - Additional Information (Detail) | Sep. 30, 2019 USD ($)
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Fair Value Disclosures [Abstract] |
Fair value, assets, level 1 to level 2 transfers, amount | $ 0
Fair value, assets, level 2 to level 1 transfers, amount | 0
Fair value, assets, level 1 to level 3 transfers, amount | 0
Fair value, assets, level 3 to level 1 transfers, amount | 0
Fair value, assets, level 2 to level 3 transfers, amount | 0
Fair value, assets, level 3 to level 2 transfers, amount | $ 0

X
- Definition Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 3. + References No definition available. + Details
Name: | mmi_FairValueAssetsLevel1ToLevel3TransfersAmount
Namespace Prefix: | mmi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Fair Value Assets Level 2 To Level 3 Transfers Amount + References No definition available. + Details
Name: | mmi_FairValueAssetsLevel2ToLevel3TransfersAmount
Namespace Prefix: | mmi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of transfers of assets measured on a recurring basis out of Level 3 of the fair value hierarchy into Level 1. + References No definition available. + Details
Name: | mmi_FairValueAssetsLevel3ToLevel1TransfersAmount
Namespace Prefix: | mmi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Fair Value Assets Level 3 To Level 2 Transfers Amount + References No definition available. + Details
Name: | mmi_FairValueAssetsLevel3ToLevel2TransfersAmount
Namespace Prefix: | mmi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
Name: | us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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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