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Common Stock (Details) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 12 Months Ended
Feb. 22, 2018
Sep. 30, 2002
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Class of Stock [Line Items]          
Dividends paid to shareholders     $ 5,965 $ 5,372 $ 5,117
Cash dividends paid (in USD per share)     $ 0.86 $ 0.78 $ 0.74
Percentage of common stock authorized for repurchase   5.00%      
Remaining shares available under stock repurchase program (in shares)     168,901    
Shares repurchased (in shares)     58,341 0  
Subsequent Event          
Class of Stock [Line Items]          
Cash dividends declared (in USD per share) $ 0.24