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Earnings Per Share and Weighted-Average Shares Outstanding - Additional Information (Detail) - shares shares in Millions | 3 Months Ended
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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