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Stockholders' Equity (Common and Preferred Stock) (Detail)
 9 Months Ended 12 Months Ended 63 Months Ended
Sep. 30, 2017
Vote
$ / shares
shares
Dec. 31, 2016
Vote
$ / shares
shares
Dec. 31, 2011
$ / shares
shares
Dec. 31, 2015
$ / shares
shares
Private Placement [Member]        
Common Stock [Abstract]        
Common stock shares issued (in shares)     20,000,000  
Sale of Stock, Price Per Share | $ / shares     $ 10  
Sale of Stock, Number of Shares Issued in Transaction     20,000,000  
Sale Of Stock Nature Of Consideration Received Per Transaction   The Company ceased raising capital under this private placement offering effective December 31, 2011.    
Preferred Stock [Member]        
Preferred Stock [Abstract]        
Preferred shares authorized (in shares) 8,990,000 8,990,000    
Convertible Series AA Preferred Stock [Member]        
Preferred Stock [Abstract]        
Preferred shares authorized (in shares)   1,000,000    
Annual cash dividend rate (in hundredths)   7.00%    
Liquidation preference per share (in dollars per share) | $ / shares   $ 25.00    
Initial conversion price (in dollars per share) | $ / shares   $ 25.00    
Total number of common shares issuable upon conversion of outstanding shares of preferred stock (in shares)   2    
Preferred shares issued (in shares)   0    
Common Class A [Member]        
Common Stock [Abstract]        
Common stock, shares authorized (in shares) 100,000,000 100,000,000   100,000,000
Par value (in dollars per share) | $ / shares $ 0.01 $ 0.01   $ 0.01
Common stock shares issued (in shares) 17,614,160 17,502,673   17,202,228
Number of votes holder is entitled to | Vote 1 1    
Maximum individual common stock ownership (in hundredths) 9.80% 9.80%    
Common Class B [Member]        
Common Stock [Abstract]        
Common stock, shares authorized (in shares) 1,000 1,000    
Par value (in dollars per share) | $ / shares $ 0.01 $ 0.01    
Common stock shares issued (in shares) 0 0    
Number of votes holder is entitled to | Vote 1 1    
Maximum individual common stock ownership (in hundredths) 9.80% 9.80%