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Schedule I - Parent Company Only Financial Statements - Statements of Cash Flows (Parenthetical) (Detail) - Parent Company [Member] - Newmark [Member] - shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Class A Common Stock [Member]      
Condensed Financial Statements Captions [Line Items]      
Common stock, shares issued 115.6 115.6 115.6
Class B Common Stock [Member]      
Condensed Financial Statements Captions [Line Items]      
Common stock, shares issued 15.8 15.8 15.8