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Shareholders' Equity (Narrative) (Details) - USD ($)
shares in Thousands, $ in Thousands
 9 Months Ended 108 Months Ended
Sep. 30, 2017
Sep. 30, 2017
Oct. 24, 2017
Equity, Class of Treasury Stock [Line Items]      
Stock repurchase program, authorized amount $ 2,300,000 $ 2,300,000  
Acquisition of treasury stock (shares) 545 7,408  
Acquisition of treasury stock (value), total $ 207,159 $ 2,204,968  
Value of common shares remaining to be repurchased $ 95,425 $ 95,425  
Subsequent Event [Member]      
Equity, Class of Treasury Stock [Line Items]      
Stock repurchase program, additional authorized amount     $ 100,000