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Shareholders' Equity - Summary of Share Count and Par Value Data Related to Shareholders' Equity (Details) - $ / shares
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Preferred Stock      
Par value per share $ 0 $ 0  
Shares authorized, shares 25,000,000 25,000,000  
Shares outstanding, shares 0 0  
Common Stock      
Par value per share $ 0 $ 0  
Shares authorized, shares 75,000,000 75,000,000  
Shares issued, shares 56,632,488 56,456,516  
Shares outstanding 32,938,466 32,762,494 32,548,477
Treasury stock      
Shares held, shares 23,694,022 23,694,022