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 9 Months Ended
Apr. 11, 2018
Feb. 29, 2016
Sep. 30, 2018
Dec. 31, 2017
Class of Stock [Line Items]        
Shares reclassified to common stock 3,500,000      
Senior common stock, authorized but unissued 57,969      
Capital stock, shares authorized 100,000,000      
Common stock, shares authorized 87,700,000   87,700,000 34,200,000
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)     500,000  
Net proceeds from offering     $ 10,100,000  
Maximum remaining capacity to sell preferred stock under open market sale agreement     $ 76,000,000  
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]        
Redeemable preferred stock, dividend rate percentage (as percent)   7.50%    
Maximum aggregate sales price of shares to be issued under open market sale agreement   $ 40,000,000.0    
Cantor Fitzgerald & Co | Preferred Stock | Series D Cumulative Redeemable Preferred Stock        
Class of Stock [Line Items]        
Maximum remaining capacity to sell preferred stock under open market sale agreement     18,600,000  
Maximum aggregate sales price of shares to be issued under open market sale agreement     $ 50,000,000.0  
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)     100,000  
Consideration received     $ 2,200,000