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Selected Balance Sheet Data - Schedule of Advances and Loans, Net (Detail) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2021 | Mar. 31, 2020
Advances And Loans and Commissions Receivable Allowance For Credit Losses [Line Items] |  |
Beginning balance | $ 563 | $ 512
Credit loss recovery | (67) | (120)
Write-offs | (2) | (2)
Ending balance | 494 | 390
Beginning balance | 94 |
Credit loss recovery | (79) | 0
Write-offs | 0 | 0
Ending balance | 15 | 32
Beginning balance | 657 |
Credit loss recovery | (146) | (120)
Write-offs | (2) | (2)
Ending balance | $ 509 | 422
Accounting Standards Update 2016-13 [Member] |  |
Advances And Loans and Commissions Receivable Allowance For Credit Losses [Line Items] |  |
Beginning balance |  | 32
Beginning balance |  | $ 544

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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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- 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 Advances and loans and commissions receivable allowance for credit loss. + References No definition available. + Details
Name: | mmi_AdvancesAndLoansAndCommissionsReceivableAllowanceForCreditLoss
Namespace Prefix: | mmi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Advances And Loans And Commissions Receivable Allowance For Credit Losses. + References No definition available. + Details
Name: | mmi_AdvancesAndLoansAndCommissionsReceivableAllowanceForCreditLossesLineItems
Namespace Prefix: | mmi_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

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- Definition Commission receivable allowance for credit loss. + References No definition available. + Details
Name: | mmi_CommissionReceivableAllowanceForCreditLoss
Namespace Prefix: | mmi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Commissions receivable allowances for credit losses write-offs. + References No definition available. + Details
Name: | mmi_CommissionsReceivableAllowancesForCreditLossesWriteOffs
Namespace Prefix: | mmi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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

X
- Definition Amount of (recovery) expense (reversal of expense) for expected credit loss for commissions receivable. + References No definition available. + Details
Name: | mmi_CreditLossRecoveryExpenseReversalForCommissionReceivable
Namespace Prefix: | mmi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201613Member
Namespace Prefix: |
Data Type: | na
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