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Summary of Significant Accounting Policies Stock-Based Compensation (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares authorized for issuance 34,300,000    
Cumulative-effect of adoption of ASU 2016-09   $ 15.7  
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) 552,215 794,764 516,475
Estimated weighted-average fair value per stock option (in dollars per share) $ 7.45 $ 5.14 $ 5.29
Risk-free interest rate, minimum (as a percent) 0.70% 0.40% 0.10%
Risk-free interest rate, maximum (as a percent) 2.50% 2.20% 2.10%
Dividend yield (as a percent) 3.50% 4.00% 3.70%
Expected volatility (as a percent) 19.00% 18.10% 18.00%
Expected life (in years) 6 years 9 months 6 years 1 month 5 years 10 months
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    
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]      
Performance units, payout ratio (as a percent) 0.00%    
Performance units | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Performance units, payout ratio (as a percent) 175.00%