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Shareholders' Equity - Dividends and Stock Repurchase Programs (Details) - USD ($)
$ / shares in Units, shares in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Oct. 31, 2017
Dividends        
Cash dividends declared (in dollars per share) $ 0.24 $ 0.24 $ 0.24  
Stock Repurchase Programs        
Authorized common stock repurchase amount       $ 100,000,000.0
Shares repurchased (in shares) 1.7 1.0 1.6  
Amount of shares repurchased $ 61,900,000 $ 41,200,000 $ 70,500,000  
Amount available to repurchase shares under stock repurchase program $ 100,000,000.0