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Stock-Based Compensation (Summary Of Nonvested Shares) (Details) - Performance Share Units - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Fully Vested Undistributed Retirement-eligible units 712,572 369,878  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Share Units, Nonvested at beginning of year 780,545    
Share Units, Granted 508,161    
Share Units, Unearned or forfeited (50,523)    
Undistributed Vested Units (342,694)    
Share Units, Nonvested at end of year 895,489 780,545  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]      
Weighted-average Fair Value per Unit, Nonvested at beginning of year $ 47.54    
Fair value of share units granted (in dollars per share) 59.16 $ 44.13 $ 52.88
Weighted-average Fair Value per Unit, Unearned or forfeited 52.50    
Weighted-average Fair Value per Unit, Earned and vested 51.65    
Weighted-average Fair Value per Unit, Nonvested at end of year $ 52.28 $ 47.54