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Shareholders' Equity (Common Stock Repurchases) (Details) - Share Repurchase Plans [Member] - USD ($)
shares in Thousands, $ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Equity, Class of Treasury Stock [Line Items]    
Acquisition of treasury stock (shares), total 219 141
Acquisition of treasury stock (value), total $ 68,038 $ 57,959