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Stock Plans (Schedule of Restricted and Restricted Stock Units Activity) (Details) - Restricted Stock And Restricted Stock Units [Member] - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Number of Shares      
Balance at beginning of year, Number of Shares 3,245,534    
Granted, Number of Shares 641,263    
Vested, Number Of Shares (1,321,620)    
Forfeited, Number of Shares (180,676)    
Balance at end of year, Number of Shares 2,384,501 3,245,534  
Weighted-Average Grant Date Fair Value Per Share      
Balance at beginning of year, Weighted-Average Grant Date Fair Value Per Share, in USD per share $ 48.45    
Granted, Weighted-Average Grant Date Fair Value Per Share, in USD per share 71.05 $ 59.38 $ 54.54
Vested, Weighted-Average Grant Date Fair Value Per Share, in USD per share 36.40    
Forfeited, Weighted-Average Grant Date Fair Value Per Share, in USD per share 59.11    
Balance at end of year, Weighted-Average Grant Date Fair Value Per Share, in USD per share $ 60.40 $ 48.45