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Selected Balance Sheet Data - Summary of Net Change in Carrying Value of Assets Held in Rabbi Trust and Deferred Compensation Liability (Detail) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
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Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020
Balance Sheet Related Disclosures [Abstract] |  |  |  |
Increase (decrease) in the carrying value of the assets held in the rabbi trust | $ 657 | $ 1,124 | $ 990 | $ (264)
Increase (decrease) in the net carrying value of the deferred compensation obligation | $ 503 | $ 973 | $ 763 | $ (300)

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- Definition The amount of increase or decrease in the fair value of plan assets during the reporting period. + References No definition available. + Details
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Namespace Prefix: | mmi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition The increase (decrease) during the reporting period in the obligation created by employee agreements whereby earned compensation will be paid in the future. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585 + Details
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