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Common Stock and Earnings Per Share - Summary of Common Stock (Details) - USD ($)
shares in Thousands, $ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Summary of Common Stock [Line Items]        
Balance, Shares 53,200 52,100    
Common Stock [Member]        
Summary of Common Stock [Line Items]        
Balance, Shares 53,220 52,085 51,679 51,509
Balance, Equity $ 1,536.7 $ 1,460.9 $ 1,436.7 $ 1,428.5
Employee Stock Purchase Plan, Shares 17 13 8  
Employee Stock Purchase Plan, Equity $ 0.8 $ 0.7 $ 0.7  
Invest Direct, Shares 263 309 38  
Invest Direct, Equity $ 17.5 $ 18.3 $ 3.0  
Options and Stock Awards, Shares 49 35 85  
Options and Stock Awards, Equity $ 3.4 $ 2.2 $ 1.3  
Contributions to RSOP, Shares 24 49 39  
Contributions to RSOP, Equity $ 3.1 $ 3.0 $ 3.2  
Common Stock Issued $ 51.0      
Equity Issuance Program, Shares 782 0 0  
Equity Issuance Program Shares Authorized     13,600  
Equity Issuance Program Shares Available for Issuance 2,100      
Common Stock [Member] | PIE Shares        
Summary of Common Stock [Line Items]        
Equity Issuance Program, Shares 800