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Stock Option Plans - Stock Option Activity (Details) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]    
Options outstanding at the beginning of the year 1,088,278 1,267,802
Options granted 313,900 171,000
Options forfeited (4,000) (33,825)
Options exercised (109,098) (316,699)
Options outstanding at the end of the year 1,289,080 1,088,278
Options exercisable at the end of the year 814,180 769,403
Common stock available for future grants at the end of the year 62,700 66,850
Weighted average exercise price per share:    
Granted $ 24.87 $ 21.41
Forfeited 19.12 15.03
Exercised 13.13 10.99
Outstanding 16.87 14.18
Exercisable $ 13.38 $ 12.29