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Earnings Per Share and Weighted-Average Shares Outstanding - Additional Information (Detail) - shares shares in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Earnings Per Share [Abstract]
Potentially securities excluded from computation of fully diluted earnings per share amount (in shares)	1.0	0.6

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- Definition

Earnings Per Share, Potentially Dilutive Securities, Shares

+ References

No definition available.

+ Details

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Namespace Prefix:	nmrk_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

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- References

No definition available.

+ Details

Name:	us-gaap_EarningsPerShareAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration