Annual report pursuant to Section 13 and 15(d)

Stock - Based Compensation - Summary of Stock Option Activity (Details)

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Stock - Based Compensation - Summary of Stock Option Activity (Details) - USD ($)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]        
Outstanding, beginning of year, weighted average exercise price (in dollars per share) $ 14.16 $ 12.96 $ 12.77  
Exercised, weighted average exercise price (in dollars per share) 12.61 12.09 11.55  
Expired, weighted average exercise price (in dollars per share)   16.01 15.26  
Outstanding, end of year, weighted average exercise price (in dollars per share) 14.26 $ 14.16 $ 12.96 $ 12.77
Exercisable, weighted average exercise price (in dollars per share) $ 14.13      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]        
Outstanding, beginning of year (in shares) 59,096 145,511 173,048  
Exercised (in shares) (3,628) (85,440) (25,632)  
Expired (in shares)   (975) (1,905)  
Outstanding, end of year (in shares) 55,468 59,096 145,511 173,048
Exercisable (in shares) 51,745      
Options outstanding, weighted average contractual term 3 years 9 months 18 days 4 years 9 months 18 days 4 years 7 months 6 days 5 years 4 months 24 days
Options exercisable, weighted average contractual term 3 years 8 months 12 days      
Options expired, intrinsic value   $ 842,742 $ 78,818 $ 0
Options outstanding, intrinsic value $ 18,331 $ 143,737 $ 156,445  
Options exercisable, intrinsic value $ 18,331      
Options available (in shares) 215,823      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest [Roll Forward]        
Expected to vest, beginning of the year, weighted average exercise price (in dollars per share) $ 15.83 $ 14.72    
Vested, weighted average exercise price (in dollars per share) 15.72 14.78    
Forfeited, weighted average exercise price (in dollars per share)   16.01    
Expected to vest, end of the year, weighted average exercise price (in dollars per share) $ 16.01 $ 15.83 $ 14.72  
Expected to vest (in shares) 9,969 25,650    
Vested (in shares) (6,246) (15,380)    
Forfeited unvested options (in shares)   (301)    
Expected to vest (in shares) 3,723 9,969 25,650  
Options expected to vest, weighted average contractual term (years) 5 years 1 month 6 days 6 years 6 years 6 months  
Options granted, weighted average fair value (in dollars per share) $ 6.11 $ 6.44 $ 6.41  
Options expected to vest, vested, grant date fair value (dollars per share) 6.46 6.38    
Stock Options        
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]        
Exercisable, weighted average exercise price (in dollars per share) $ 14.13 $ 13.82 $ 12.45  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]        
Exercised (in shares) (3,628) (85,440)    
Exercisable (in shares) 51,745 49,127 119,861  
Options available, expired   975 1,905  
Options available (in shares) 155,845 155,845 154,870 152,965