Quarterly report pursuant to Section 13 or 15(d)

Stockholders' Equity - Additional Information (Detail)

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Stockholders' Equity - Additional Information (Detail) - USD ($)
 1 Months Ended 3 Months Ended 9 Months Ended
Mar. 28, 2015
Mar. 31, 2011
Mar. 31, 2015
Sep. 30, 2015
Sep. 30, 2014
Dec. 31, 2014
Class of Stock [Line Items]            
Common stock, shares issued       21,743,779   19,589,606
Common stock, par value       $ 0.001   $ 0.001
Shares intended to be offered pursuant to primary offering   3,000,000        
Shares offered under distribution reinvestment plan   500,000        
Shares of senior common stock sold in offering 927,994   189,052      
Senior common stock offering, gross proceeds       $ 39,495,000 $ 42,636,000  
Senior Common Stock [Member]            
Class of Stock [Line Items]            
Common stock, shares issued   3,500,000        
Common stock, par value   $ 0.001        
Common stock shares issued, selling price   $ 15.00 $ 15.00      
Shares offered under distribution reinvestment plan 27,038   5,134      
Net proceeds after deducting underwriting discount and commission     $ 2,600,000      
Senior common stock offering, gross proceeds $ 13,900,000          
Cantor Fitzgerald & Co [Member]            
Class of Stock [Line Items]            
Maximum aggregate sales price of shares to be issued under open market sale agreement         $ 100,000,000  
Net proceeds under ATM program       $ 36,200,000    
Number of shares sold under open market sale agreement       4,000,000    
Gross proceeds of shares sold under open market sale agreement       $ 69,300,000    
Maximum remaining capacity to sell common stock under open market sale agreement       $ 30,700,000