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Marketable Securities - Additional Information (Detail) - USD ($)
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 28, 2013
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Equity [Line Items]            
Gross proceeds from sale of marketable securities       $ 22,204,000 $ 51,433,000  
Gain (loss) on marketable securities   $ 1,900,000 $ 900,000 1,812,000 1,494,000  
Marketable securities, unrealized gain (loss) on mark to market securities       5,110,000 1,444,000  
Marketable securities   33,659,000   33,659,000   $ 48,942,000
Other Income (Loss), Net [Member]            
Equity [Line Items]            
Marketable securities, unrealized gain (loss) on mark to market securities   $ 1,200,000 $ 600,000 $ 5,100,000 $ 1,500,000  
Nasdaq [Member]            
Equity [Line Items]            
Number of shares sold in transaction       250,000    
Remaining number of earn-out shares received under common stock transaction   350,000   350,000    
Certain Assets of Nasdaq [Member]            
Equity [Line Items]            
Period for earn-out receivable under common stock transaction       9 years    
Amount recognized in connection with the earn-out including other income (loss)           992,247
Certain Assets of Nasdaq [Member] | BGC Partners Inc [Member]            
Equity [Line Items]            
Period for earn-out receivable under common stock transaction 15 years          
Maximum [Member] | Certain Assets of Nasdaq [Member]            
Equity [Line Items]            
Remaining earn-out receivable under common stock transaction       8,930,223    
Maximum [Member] | Certain Assets of Nasdaq [Member] | NEWMARK Group Inc Parent [Member]            
Equity [Line Items]            
Earn-out shares receivable under common stock transaction 14,883,705          
Minimum [Member] | Certain Assets of Nasdaq [Member]            
Equity [Line Items]            
Gross revenue on earn-out receivable per year under common stock transaction       $ 25,000,000.0    
Minimum [Member] | Certain Assets of Nasdaq [Member] | NEWMARK Group Inc Parent [Member]            
Equity [Line Items]            
Gross revenue on earn-out receivable per year under common stock transaction $ 25,000,000.0