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Income Taxes - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Taxes [Line Items]
Provision for income taxes, rate	28.80%	31.20%

X

- Definition

Income Taxes [Line Items]

+ References

No definition available.

+ Details

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Namespace Prefix:	mmi_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration