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Stockholders' Equity (Dividends) (Detail) - USD ($)
 9 Months Ended 12 Months Ended
Jan. 23, 2012
Jan. 22, 2012
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Dec. 31, 2015
Cash Dividends [Abstract]            
Cash dividends paid, net of reinvested stock dividends     $ 3,621,000 $ 3,348,000 $ 4,517,000 $ 4,124,000
Annualized dividend rate (in dollars per share)     $ 0.10   $ 0.40  
Dividend Reinvestment Plan [Abstract]            
Registered shares of common stock pursuant to dividend reinvestment plan (in shares) 3,000,000          
Percentage of purchase price per share (in hundredths) 95.00%          
Former sales price per share (in dollars per share) $ 9.50 $ 10.00        
Payment of sales commission or dealer manager fee $ 0          
Notice period for amendments to Dividend Reinvestment Plan     30 days   30 days  
Common stock issued under dividend reinvestment plan     $ 17,400,000   $ 16,325,429  
Common stock issued under dividend reinvestment plan (in shares)     1,834,147   1,718,466