| 
           Stock-Based Compensation Plans - Summary of SARs Activity - Pre-IPO (Detail) 
         | 
        12 Months Ended | ||
|---|---|---|---|
| 
           Dec. 31, 2013 
         | 
        
           Dec. 31, 2012 
         | 
        
           Dec. 31, 2014 
         | 
      |
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | |||
| Outstanding, beginning balance, Shares | 1,207,009us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber | ||
| Outstanding, ending balance, Shares | 913,915us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber | 1,207,009us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber | |
| SARs [Member] | |||
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | |||
| Outstanding, beginning balance, Shares | 
28,733us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember  | 
        
27,983us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember  | 
        |
| Granted | 
750us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember  | 
        ||
| Settled | 
(28,733)mmi_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsSettledInPeriod / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember  | 
        ||
| Outstanding, ending balance, Shares | 
28,733us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember  | 
        ||
| Outstanding, ending balance, Shares | 
22,666mmi_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVested / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember  | 
        ||
| X | ||||||||||
| 
             
- Definition 
          Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Settled In Period No definition available. 
  | 
        
| X | ||||||||||
| 
             
- Definition 
          Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested No definition available. 
  | 
        
| X | ||||||||||
| 
             
- Definition 
          The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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| X | ||||||||||
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- Details 
          
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| X | ||||||||||
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- Definition 
          Gross number of share options (or share units) granted during the period. No definition available. 
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| X | ||||||||||
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- Details 
          
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