Annual report pursuant to Section 13 and 15(d)

Stockholders' Equity and Mezzanine Equity - Additional Information (Detail)

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Stockholders' Equity and Mezzanine Equity - Additional Information (Detail) - USD ($)
 1 Months Ended 3 Months Ended 12 Months Ended
Mar. 28, 2015
Jul. 31, 2017
Feb. 29, 2016
Mar. 31, 2011
Mar. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jun. 30, 2016
Class of Stock [Line Items]                  
Common stock, shares issued (in shares)           28,384,016 24,882,758    
Common stock, par value (in dollars per share)           $ 0.001 $ 0.001    
Shares intended to be offered pursuant to primary offering (in shares)       3,000,000          
Shares intended to be offered pursuant to senior common distribution reinvestment plan (in shares)       500,000          
Shares of senior common stock sold in offering (in shares) 927,994       189,052        
Senior common stock offering, gross proceeds           $ 86,260,000 $ 118,362,000 $ 50,165,000  
Series D Cumulative Redeemable Preferred Stock                  
Class of Stock [Line Items]                  
Minimum vote need to trigger change in control from tender offer (as percent)           90.00%      
Senior Common Stock                  
Class of Stock [Line Items]                  
Common stock, shares issued (in shares)       3,500,000          
Common stock, par value (in dollars per share)       $ 0.001          
Common stock shares issued, selling price (in dollars per share)       $ 15.00 $ 15.00        
Shares intended to be offered pursuant to senior common distribution reinvestment plan (in shares) 27,038       5,134        
Net proceeds after deducting underwriting discount and commission         $ 2,600,000        
Senior common stock offering, gross proceeds $ 13,900,000                
Common Stock                  
Class of Stock [Line Items]                  
Offering price (in dollars per share)   $ 20.52              
Overallotment sale of stock, price per share   $ 3,400,000              
Senior common stock offering, gross proceeds   $ 22,500,000              
Common Stock | Cantor Fitzgerald & Co                  
Class of Stock [Line Items]                  
Maximum aggregate sales price of shares to be issued under open market sale agreement     $ 160,000,000            
Number of shares sold under open market sale agreement (in shares)           2,133,000      
Preferred Stock | Series D Cumulative Redeemable Preferred Stock                  
Class of Stock [Line Items]                  
Preferred stock, dividend rate (as percent)     7.00%            
Preferred Stock | Cantor Fitzgerald & Co                  
Class of Stock [Line Items]                  
Remaining capacity to sell Preferred stock           $ 37,200,000      
Preferred Stock | Cantor Fitzgerald & Co | Series A Cumulative Redeemable Preferred Stock                  
Class of Stock [Line Items]                  
Preferred stock, dividend rate (as percent)     7.75%            
Preferred Stock | Cantor Fitzgerald & Co | Series B Cumulative Redeemable Preferred Stock                  
Class of Stock [Line Items]                  
Net proceeds from offering     $ 40,000,000            
Preferred stock, dividend rate (as percent)     7.50%            
Preferred Stock | Cantor Fitzgerald & Co | 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
Registered Public Offering | Common Stock                  
Class of Stock [Line Items]                  
Number of shares issued (in shares)   1,200,000              
Sale of stock, number of shares issued in overallotment   200,000              
Series D Preferred Stock ATM Program | Cantor Fitzgerald & Co | Series D Cumulative Redeemable Preferred Stock                  
Class of Stock [Line Items]                  
Number of shares issued (in shares)           500,000      
Net proceeds from sale of shares           $ 12,700,000      
Cantor Fitzgerald & Co | Common Stock                  
Class of Stock [Line Items]                  
Net proceeds from offering           45,500,000      
Remaining capacity to sell Preferred stock           86,300,000      
Cantor Fitzgerald & Co | Preferred Stock | Series D Cumulative Redeemable Preferred Stock                  
Class of Stock [Line Items]                  
Remaining capacity to sell Preferred stock           $ 20,800,000