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Stockholders' Equity and Stock Plans - Dividends (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Equity [Abstract]        
Cash dividends to common stockholders $ 12,506 $ 10,958 $ 8,860 [1]  
Cash dividends per common share (usd per share) $ 0.92 $ 0.80 $ 0.64  
Retained earnings $ 219,747 $ 203,227    
Stockholders' equity $ 358,253 $ 336,788 $ 316,407 $ 297,025
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 $ 30,600      
Cash held $ 5,300      
[1] Share and per share data have been adjusted to reflect the two-for-one stock split effective November 27, 2018.