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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, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Summary of Common Stock [Line Items]        
Balance, Shares 51,100 49,600    
Received for Sale of Land Inventory, Shares   (100)    
Received for Sale of Land Inventory, Equity   $ (8.0)    
Acquisition of Non-Controlling Interest, Equity   (6.7)    
Employer Contributions to Pension - Value $ 13.5 $ 0.0 $ 0.0  
Common Stock [Member]        
Summary of Common Stock [Line Items]        
Balance, Shares 51,117 49,560 49,075 45,929
Balance, Equity $ 1,401.4 $ 1,295.3 $ 1,271.4 $ 1,107.6
Employee Stock Purchase Plan, Shares 12 16 18  
Employee Stock Purchase Plan, Equity $ 0.8 $ 0.9 $ 0.9  
Invest Direct, Shares 257 344 383  
Invest Direct, Equity $ 19.0 $ 20.0 $ 19.0  
Options and Stock Awards, Shares 22 65 43  
Options and Stock Awards, Equity $ 3.6 $ 3.7 $ 8.6  
Equity Issuance Program, Shares 1,000 130 1,289  
Equity Issuance Program, Equity $ 65.7 $ 8.0 $ 69.9  
Forward Sale Agreement and Issuance, Shares     1,413  
Forward Sales Agreement and Issuance, Equity     $ 65.4  
Contributions to RSOP, Shares 50 60    
Contributions to RSOP, Equity $ 3.5 $ 3.3    
Received for Sale of Land Inventory, Shares   (130)    
Received for Sale of Land Inventory, Equity   $ (8.0)    
Acquisition of Non-Controlling Interest, Shares   0    
Acquisition of Non-Controlling Interest, Equity   $ 4.0    
Employer Contributions to Pension - Shares 216 0    
Employer Contributions to Pension - Value $ 13.5      
Equity Issuance Program Shares Authorized   13,600    
Equity Issuance Program Shares Available for Issuance   2,900    
Antidilutive Options to Purchase Shares of Common Stock Excluded from the Computation of Earnings Per Share 0 0 0