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Short-Term and Long-Term Debt - Long-Term Debt (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Debt Instrument [Line Items]    
Long-Term Debt $ 1,370.4 $ 1,556.7
Long-Term Debt - Unamortized Debt Issuance Costs 10.4  
Long-term Debt Maturities [Abstract]    
Long-Term Debt Maturing in 2017 188.3  
Long-Term Debt Maturing in 2018 63.1  
Long-Term Debt Maturing in 2019 55.2  
Long-Term Debt Maturing in 2020 101.2  
Long-Term Debt Maturing in 2021 96.4  
Long-Term Debt Maturing Thereafter 1,064.9  
Camp Ripley Financing [Member]    
Long-term Debt Maturities [Abstract]    
Annual Financing Payment $ 1.4  
Financing Renewal Term 2 years  
Purchase Option $ 4.0  
Senior Unsecured Notes 3.11% Due 2027 [Member]    
Long-term Debt Maturities [Abstract]    
Expected Proceeds from Future Issuance of Senior Unsecured Notes $ 80.0  
Expected Future Interest Rate 3.11%