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Stock Transactions and Unit Redemptions - Additional Information (Detail) - USD ($)
3 Months Ended 12 Months Ended
Apr. 12, 2017
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jun. 22, 2016
Jun. 21, 2016
Oct. 27, 2015
Class of Stock [Line Items]                      
Payment percentage of the gross proceeds from the sale of shares           2.00%          
Stock repurchase program and unit redemption remaining authorized amount   $ 196,577,825       $ 196,577,825          
Redemptions, Number of Units   1,984,933 1,675,330 1,908,867 2,927,758 8,496,888          
Reduction cost due to redemption and repurchases of shares           $ 35,200,000          
Reduction cost due to redemption and repurchases of shares per share           $ 12.00          
Newmark Holdings [Member]                      
Class of Stock [Line Items]                      
Redemptions, Number of Units           0          
Class A Common Stock [Member]                      
Class of Stock [Line Items]                      
Common stock, shares authorized   750,000,000       750,000,000 750,000,000   750,000,000 500,000,000  
Common stock, issued during the period           1,994,670 2,004,533 129,151      
Common stock outstanding   256,968,000       256,968,000 244,870,000        
Maximum number of shares under the controlled equity offering           20,000,000          
Common stock, shares issued   306,218,000       306,218,000 292,549,000        
Company's repurchase and additional redemption authority amount                     $ 300,000,000
Stock repurchase program and unit redemption remaining authorized amount   $ 196,600,000       $ 196,600,000          
Reduction in diluted shares due to redemption and repurchases of shares           2,900,000          
Class A Common Stock [Member] | Controlled Equity Offerings [Member]                      
Class of Stock [Line Items]                      
Common stock, shares issued   20,000,000       20,000,000          
Redemptions, Number of Units           7,000,000          
Class A Common Stock [Member] | April 2017 Sales Agreement                      
Class of Stock [Line Items]                      
Maximum number of shares under the controlled equity offering 20,000,000                    
Common stock, shares issued   2,775,485       2,775,485          
Class B Common Stock [Member]                      
Class of Stock [Line Items]                      
Common stock, shares authorized   150,000,000       150,000,000 150,000,000   150,000,000 100,000,000  
Common stock, issued during the period           0 0        
Common stock outstanding   34,848,107       34,848,107 34,848,107        
Common stock, shares issued   34,848,000       34,848,000 34,848,000