Quarterly report pursuant to Section 13 or 15(d)

Stockholders' and Mezzanine Equity - Additional Information (Detail)

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Stockholders' and Mezzanine Equity - Additional Information (Detail) - USD ($)
 1 Months Ended 3 Months Ended
Feb. 29, 2016
Mar. 31, 2018
Dec. 31, 2017
Jun. 30, 2016
Series A and B Preferred Stock ATM Programs        
Class of Stock [Line Items]        
Redeemable preferred stock, shares issued (in shares)   2,264,000 2,264,000  
Series D Cumulative Redeemable Preferred Stock        
Class of Stock [Line Items]        
Redeemable preferred stock, dividend rate percentage (as percent)   7.00%    
Temporary equity, contract terms, minimum vote needed to trigger change in control from tender offer (as percent)   90.00%    
Cantor Fitzgerald & Co | Common Stock        
Class of Stock [Line Items]        
Redeemable preferred stock, shares issued (in shares)   30,000    
Net proceeds from offering   $ 600,000    
Maximum remaining capacity to sell preferred stock under open market sale agreement   $ 85,600,000    
Cantor Fitzgerald & Co | Preferred Stock | Series A and B Preferred Stock ATM Programs        
Class of Stock [Line Items]        
Redeemable preferred stock, shares issued (in shares)   0    
Maximum remaining capacity to sell preferred stock under open market sale agreement   $ 37,200,000    
Cantor Fitzgerald & Co | Preferred Stock | Series A Cumulative Redeemable Preferred Stock        
Class of Stock [Line Items]        
Redeemable preferred stock, dividend rate percentage (as percent) 7.75%      
Cantor Fitzgerald & Co | Preferred Stock | Series B Cumulative Redeemable Preferred Stock        
Class of Stock [Line Items]        
Maximum aggregate sales price of shares to be issued under open market sale agreement $ 40,000,000.0      
Redeemable preferred stock, dividend rate percentage (as percent) 7.50%      
Cantor Fitzgerald & Co | Preferred Stock | Series D Cumulative Redeemable Preferred Stock        
Class of Stock [Line Items]        
Maximum aggregate sales price of shares to be issued under open market sale agreement       $ 50,000,000.0
Maximum remaining capacity to sell preferred stock under open market sale agreement   $ 19,900,000    
Cantor Fitzgerald & Co | Series D Preferred Stock ATM Program | Series D Cumulative Redeemable Preferred Stock        
Class of Stock [Line Items]        
Number of shares issued (in shares)   40,000    
Consideration received   $ 900,000