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Summary of Significant Accounting Policies Stock Based Compensation (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share contractual period 10 years      
Number of shares available for future issuances 6,900,000      
Maximum Number of Common Stock Shares Available Under 2016 Stock Option Plan 15,000,000.0      
Award vesting rate, within one year of grant date 10.00%      
Award vesting rate, between year one and year two 15.00%      
Award vesting rate, between year two and year three 20.00%      
Award vesting rate, between year three and year four 25.00%      
Award vesting rate, between year four and year five 30.00%      
Expected volatility 17.00% 18.00% 20.00%  
Share-based compensation expense $ 2.1 $ 3.7 $ 3.8  
Income tax benefit $ 0.4 $ 0.7 $ 0.8  
Expected dividends 5.72% 4.10% 4.03%  
Expected term (in years) 7 years 7 years 7 years  
Risk-free rate 0.72% 2.54% 2.81%  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]        
Options, outstanding at beginning of year 8,009,237 7,163,567 6,565,019  
Granted 1,901,100 1,777,500 1,539,500  
Exercised 397,653 848,923 881,917  
Forfeited and canceled 18,033 82,907 59,035  
Options, outstanding at end of year 9,494,651 8,009,237 7,163,567  
Exercisable at end of year 6,138,602 5,100,009 4,556,350  
Options, outstanding, weighted average exercise price $ 18.36 $ 18.43 $ 17.24 $ 15.76
Granted, Weighted Average Exercise Price 17.24 21.14 20.98  
Exercised, Weighted Average Exercise Price 14.44 14.14 12.86  
Forfeited and canceled, Weighted Average Exercise Price 16.72 17.05 16.54  
Exercisable at end of year, Weighted Average Exercise Price 17.81 17.18 15.83  
Weighted average fair value of options granted during the year [1] $ 0.88 $ 2.35 $ 2.71  
Total unrecognized compensation cost $ 2.7      
Unrecognized stock compensation costs, weighted average remaining contractual term 3 years      
Cash received from stock option exercise $ 5.7 $ 12.0 $ 11.3  
Intrinsic value of stock options exercised 2.8 6.8 7.5  
Actual tax benefit realized for tax deductions from stock options exercised $ 0.5 $ 1.4 $ 1.5  
Restricted Stock [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]        
Restricted common stock vesting period 3 years      
Range of Exercise Prices, $12.33 [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]        
Options, outstanding at end of year 154,620      
Exercisable at end of year 154,620      
Options, outstanding, weighted average exercise price $ 12.33      
Exercisable at end of year, Weighted Average Exercise Price $ 12.33      
Options, Outstanding, Weighted Average Remaining Contractual Life 3 months      
Range of Exercise Prices, $10.80 [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]        
Options, outstanding at end of year 272,441      
Exercisable at end of year 272,441      
Options, outstanding, weighted average exercise price $ 10.80      
Exercisable at end of year, Weighted Average Exercise Price $ 10.80      
Options, Outstanding, Weighted Average Remaining Contractual Life 1 year 3 months      
Range of Exercise Prices, $12.57 [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]        
Options, outstanding at end of year 357,700      
Exercisable at end of year 357,700      
Options, outstanding, weighted average exercise price $ 12.57      
Exercisable at end of year, Weighted Average Exercise Price $ 12.57      
Options, Outstanding, Weighted Average Remaining Contractual Life 2 years 3 months      
Range of Exercise Prices, $16.06 [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]        
Options, outstanding at end of year 614,442      
Exercisable at end of year 614,442      
Options, outstanding, weighted average exercise price $ 16.06      
Exercisable at end of year, Weighted Average Exercise Price $ 16.06      
Options, Outstanding, Weighted Average Remaining Contractual Life 3 years 3 months      
Range of Exercise Prices, $15.26 [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]        
Options, outstanding at end of year 658,479      
Exercisable at end of year 658,479      
Options, outstanding, weighted average exercise price $ 15.26      
Exercisable at end of year, Weighted Average Exercise Price $ 15.26      
Options, Outstanding, Weighted Average Remaining Contractual Life 4 years 3 months      
Range of Exercise Prices, $18.14 [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]        
Options, outstanding at end of year 1,004,277      
Exercisable at end of year 1,004,277      
Options, outstanding, weighted average exercise price $ 18.14      
Exercisable at end of year, Weighted Average Exercise Price $ 18.14      
Options, Outstanding, Weighted Average Remaining Contractual Life 5 years 3 months      
Range of Exercise Prices, $19.98 [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]        
Options, outstanding at end of year 1,272,599      
Exercisable at end of year 990,510      
Options, outstanding, weighted average exercise price $ 19.98      
Exercisable at end of year, Weighted Average Exercise Price $ 19.98      
Options, Outstanding, Weighted Average Remaining Contractual Life 6 years 3 months      
Range of Exercise Prices, $20.98 [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]        
Options, outstanding at end of year 1,500,338      
Exercisable at end of year 864,707      
Options, outstanding, weighted average exercise price $ 20.98      
Exercisable at end of year, Weighted Average Exercise Price $ 20.98      
Options, Outstanding, Weighted Average Remaining Contractual Life 7 years 3 months      
Range of Exercise Prices, $21.12 to $21.99 [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]        
Options, outstanding at end of year 1,761,255      
Exercisable at end of year 738,395      
Options, outstanding, weighted average exercise price $ 21.14      
Exercisable at end of year, Weighted Average Exercise Price $ 21.13      
Options, Outstanding, Weighted Average Remaining Contractual Life 8 years 3 months      
Range of Exercise Prices, $16.17 to $22.72 [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]        
Options, outstanding at end of year 1,898,500      
Exercisable at end of year 483,031      
Options, outstanding, weighted average exercise price $ 17.24      
Exercisable at end of year, Weighted Average Exercise Price $ 17.19      
Options, Outstanding, Weighted Average Remaining Contractual Life 9 years 3 months      
[1] Based on the Black-Scholes-Merton option pricing model and the assumptions outlined above.