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
Feb. 29, 2016
Mar. 31, 2011
Mar. 31, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 05, 2016
Jun. 30, 2016
Class of Stock [Line Items]                  
Senior common stock offering, gross proceeds         $ 118,362,000 $ 50,165,000 $ 73,931,000    
Common stock, shares issued (in shares)         24,882,758 22,485,607      
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          
Series D Cumulative Redeemable Preferred Stock [Member]                  
Class of Stock [Line Items]                  
Preferred stock, dividend rate (as percent)         7.00%        
Par value (in dollars per share)         $ 0.001        
Net proceeds from sale of shares         $ 54,700,000        
Payment to redeem Preferred stock         $ 38,500,000        
Minimum vote need to trigger change in control from tender offer (as percent)         90.00%        
Senior Common Stock [Member]                  
Class of Stock [Line Items]                  
Senior common stock offering, gross proceeds $ 13,900,000                
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          
Common Stock [Member]                  
Class of Stock [Line Items]                  
Offering price (in dollars per share)               $ 18.35  
Senior common stock offering, gross proceeds         $ 13,900,000        
Number of shares sold under open market sale agreement (in shares)         1,558,000        
Common Stock [Member] | Cantor Fitzgerald & Co [Member]                  
Class of Stock [Line Items]                  
Number of shares sold under open market sale agreement (in shares)         65,000        
Net proceeds under ATM program         $ 900,000        
Maximum aggregate sales price of shares to be issued under open market sale agreement   $ 160,000,000              
Remaining capacity to sell Preferred stock         132,500,000        
Net proceeds from offering         27,100,000        
Preferred Stock [Member] | Cantor Fitzgerald & Co [Member]                  
Class of Stock [Line Items]                  
Remaining capacity to sell Preferred stock         $ 37,200,000        
Preferred Stock [Member] | Cantor Fitzgerald & Co [Member] | Series A Cumulative Redeemable Preferred Stock [Member]                  
Class of Stock [Line Items]                  
Preferred stock, dividend rate (as percent)   7.75%              
Preferred Stock [Member] | Cantor Fitzgerald & Co [Member] | Series B Cumulative Redeemable Preferred Stock [Member]                  
Class of Stock [Line Items]                  
Maximum aggregate sales price of shares to be issued under open market sale agreement   $ 40,000,000              
Preferred stock, dividend rate (as percent)   7.50%              
Preferred stock, shares issued (in shares)         114,000        
Net proceeds from offering         $ 2,800,000        
Preferred Stock [Member] | Cantor Fitzgerald & Co [Member] | Series D Cumulative Redeemable Preferred Stock [Member]                  
Class of Stock [Line Items]                  
Maximum aggregate sales price of shares to be issued under open market sale agreement                 $ 50,000,000.0
Remaining capacity to sell Preferred stock         $ 33,700,000        
Registered Public Offering [Member] | Common Stock [Member]                  
Class of Stock [Line Items]                  
Number of shares issued (in shares)         774,400        
Private Placement [Member] | Series D Cumulative Redeemable Preferred Stock [Member]                  
Class of Stock [Line Items]                  
Number of shares issued (in shares)         2,273,725        
Liquidation preference (in dollars per share)         $ 25.00        
Series D Preferred Stock ATM Program [Member] | Series D Cumulative Redeemable Preferred Stock [Member]                  
Class of Stock [Line Items]                  
Number of shares issued (in shares)         644,000        
Net proceeds from sale of shares         $ 16,000,000