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Stockholders' Equity and Stock Plans - Dividends (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Equity [Abstract]        
Cash dividends to common stockholders $ 6,896 $ 6,223 $ 5,390  
Cash dividends per common share (usd per share) $ 1.12 $ 1.02 $ 0.90  
Retained earnings $ 155,544 $ 146,464    
Stockholders' equity $ 297,025 $ 230,563 $ 214,473 $ 200,026
Period used to determine amount available for payment of dividends based on restriction (in years) 3 years      
Amount of retained earnings available for payment of dividends based on restriction $ 39,900      
Cash held $ 3,200