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Stock-Based Compensation - Plan and Expense Information (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Stock-Based Compensation          
Common stock reserved for future grants (in shares) 101,989   101,989    
Compensation cost recognized $ 6,000 $ 6,600 $ 17,500 $ 18,400  
Unrecognized compensation cost $ 23,000   $ 23,000    
Period during which compensation cost is expected to be recognized     3 years    
Proceeds from exercise of stock options     $ 1,885 $ 3,765  
Maximum          
Stock-Based Compensation          
Number of shares of common stock authorized for issuance 442,750   442,750    
Stock Options.          
Stock-Based Compensation          
Outstanding options (in shares) 18,168   18,168   16,168
Exercise of stock options and related tax benefit (in shares)     250 500  
Proceeds from exercise of stock options     $ 2,000 $ 4,000  
Stock Options. | Minimum          
Stock-Based Compensation          
Price range of stock award (as a percent)     85.00%    
Period over which options become exercisable     1 year    
Stock Options. | Maximum          
Stock-Based Compensation          
Price range of stock award (as a percent)     110.00%    
Expiration period of options granted     7 years    
Period over which options become exercisable     3 years