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Stock-Based Compensation (Summary Of Nonvested Shares) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Performance Share Units | Market Condition PSUs      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Percentage of shares issued per share unit, minimum 0.00%    
Percentage of shares issued per share unit, maximum 200.00%    
Fully Vested Undistributed Retirement-eligible units (in shares) 366,243 503,283  
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 38 months    
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 (in shares) 539,877    
Share Units, Granted (in shares) 267,154    
Share Units, Unearned or forfeited (in shares) (16,303)    
Share Units, Undistributed Vested Units (in shares) (178,802)    
Share Units, Vested and distributed (in shares) (147,787)    
Share Units, Performance Adjustment (in shares) 0    
Share Units, Nonvested at end of year (in shares) 464,139 539,877  
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 (in dollars per share) $ 63.79    
Fair value of share units awarded 82.49 $ 67.42 $ 62.88
Weighted-average Fair Value per Unit, Unearned or forfeited (in dollars per share) 73.42    
Weighted-average Fair Value per Unit, Vested and undistributed (in dollars per share) 69.87    
Weighted-average Fair Value per Unit, Vested and distributed (in dollars per share) 59.16    
Weighted-average Fair Value per Unit, Performance Adjustment (in dollars per share) 0    
Weighted-average Fair Value per Unit, Nonvested at end of year (in dollars per share) $ 73.34 $ 63.79  
Performance Share Units | Performance Condition PSUs      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Percentage of shares issued per share unit, minimum 0.00%    
Percentage of shares issued per share unit, maximum 200.00%    
Fully Vested Undistributed Retirement-eligible units (in shares) 7,607    
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 38 months    
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 (in shares) 0    
Share Units, Granted (in shares) 43,052    
Share Units, Unearned or forfeited (in shares) (1,155)    
Share Units, Undistributed Vested Units (in shares) (7,607)    
Share Units, Vested and distributed (in shares) 0    
Share Units, Performance Adjustment (in shares) (2,394)    
Share Units, Nonvested at end of year (in shares) 31,896 0  
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 (in dollars per share) $ 0    
Fair value of share units awarded 76.69    
Weighted-average Fair Value per Unit, Unearned or forfeited (in dollars per share) 76.80    
Weighted-average Fair Value per Unit, Vested and undistributed (in dollars per share) 76.78    
Weighted-average Fair Value per Unit, Vested and distributed (in dollars per share) 0    
Weighted-average Fair Value per Unit, Performance Adjustment (in dollars per share) 76.66    
Weighted-average Fair Value per Unit, Nonvested at end of year (in dollars per share) $ 76.66 $ 0  
Restricted Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Fully Vested Undistributed Retirement-eligible units (in shares) 160,034 79,854  
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 38 months    
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 (in shares) 220,277    
Share Units, Granted (in shares) 173,571    
Share Units, Unearned or forfeited (in shares) (7,507)    
Share Units, Undistributed Vested Units (in shares) (80,180)    
Share Units, Vested and distributed (in shares) (2,466)    
Share Units, Performance Adjustment (in shares) 0    
Share Units, Nonvested at end of year (in shares) 303,695 220,277  
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 (in dollars per share) $ 61.13    
Fair value of share units awarded 76.91    
Weighted-average Fair Value per Unit, Unearned or forfeited (in dollars per share) 69.89    
Weighted-average Fair Value per Unit, Vested and undistributed (in dollars per share) 66.72    
Weighted-average Fair Value per Unit, Vested and distributed (in dollars per share) 54.30    
Weighted-average Fair Value per Unit, Performance Adjustment (in dollars per share) 0    
Weighted-average Fair Value per Unit, Nonvested at end of year (in dollars per share) $ 68.52 $ 61.13  
Restricted Stock Units | September 18, 2020 [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Granted in Period, Fair Value $ 3    
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 3 years    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Share Units, Granted (in shares) 37,104