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Summary of Significant Accounting Policies Stock-Based Compensation (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of shares authorized for issuance 34,300,000      
Stock options        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period (in years) 3 years      
Minimum exercise price of stock option as a percent of common stock fair value on the grant date 100.00%      
Period after the grant date during which stock options can't be exercised (in months) 6 months      
Maximum term of awards (in years) 10 years      
Stock options granted (in shares) 476,418 710,710 552,215  
Estimated weighted-average fair value per stock option (in dollars per share) $ 8.60 $ 7.71 $ 7.45  
Risk-free interest rate, minimum (as a percent) 2.50% 1.60% 0.70%  
Risk-free interest rate, maximum (as a percent) 2.70% 2.80% 2.50%  
Dividend yield (as a percent) 3.60% 3.50% 3.50%  
Expected volatility (as a percent) 17.00% 18.00% 19.00%  
Expected life (in years) 8 years 6 months 5 years 10 months 24 days 6 years 9 months 18 days  
Restricted stock | Employees        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period (in years) 3 years      
Percentage to vest each year after grant date 33.00%      
Restricted stock | Directors | Granted prior to January 1, 2017        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period (in years) 3 years      
Percentage to vest each year after grant date 33.00%      
Restricted stock | Directors | Granted after January 1, 2017        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period (in years) 1 year      
Restricted stock | Certain Officers | Granted after January 1, 2017        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period (in years) 1 year      
Performance units        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period (in years) 3 years      
Performance units | Minimum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Payout ratio (as a percent) 0.00%      
Performance units | Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Payout ratio (as a percent) 175.00%      
ASU 2016-09        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Cumulative effect adjustment from new accounting principle       $ 15.7