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Selected Balance Sheet Data - Schedule of Advances and Loans, Net (Detail) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2020 | Mar. 31, 2019
Advances And Loans Allowance For Credit Losses [Line Items] |  |
Beginning balance | $ 512 | $ 514
Credit loss recovery | (120) | (104)
Write-offs | (2) | (3)
Ending balance | $ 390 | $ 407

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- Definition Advances and loans allowance for credit losses. + References No definition available. + Details
Name: | mmi_AdvancesAndLoansAllowanceForCreditLosses
Namespace Prefix: | mmi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
Name: | mmi_AdvancesAndLoansAllowanceForCreditLossesLineItems
Namespace Prefix: | mmi_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Advances and loans allowance for credit losses write offs. + References No definition available. + Details
Name: | mmi_AdvancesAndLoansAllowanceForCreditLossesWriteOffs
Namespace Prefix: | mmi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Amount of expense (reversal of expense) for expected credit loss for advances and loans. + References No definition available. + Details
Name: | mmi_CreditLossExpenseReversalForAdvancesAndLoans
Namespace Prefix: | mmi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration