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Debt (Schedule of Short-Term and Long-Term Debt) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Long-term Debt    
Long-term debt $ 2,231.2 $ 2,229.3
Seven Hundred Fifty Million Fixed Rate Notes Due March Two Thousand Twenty Two, Interest Equal To Four Point Three Zero Percent [Member]    
Long-term Debt    
Long-term debt [1] $ 745.2 744.4
Forward starting interest rate swap agreement fixed rate 3.32%  
Debt instrument, face amount $ 750.0  
Debt Instrument, maturity date Sep. 01, 2022  
Debt Instrument, Interest Rate, Stated Percentage 3.00%  
Seven Hundred Fifty Million Fixed Rate Notes Due March Two Thousand Twenty Five, Interest Equal To Three Point Zero Percent [Member] [Member]    
Long-term Debt    
Long-term debt [1] $ 744.2 743.4
Forward starting interest rate swap agreement fixed rate 3.11%  
Debt instrument, face amount $ 750.0  
Debt Instrument, maturity date Mar. 01, 2025  
Debt Instrument, Interest Rate, Stated Percentage 3.00%  
Seven Hundred Fifty Million Fixed Rate Notes Due March Two Thousand Forty Three, Interest Equal To Five Point Three Percent [Member]    
Long-term Debt    
Long-term debt [1] $ 741.8 $ 741.5
Forward starting interest rate swap agreement fixed rate 4.73%  
Debt instrument, face amount $ 750.0  
Debt Instrument, maturity date Sep. 01, 2043  
Debt Instrument, Interest Rate, Stated Percentage 5.30%  
[1] In August 2012, the company entered into a forward-starting interest rate swap agreement that modified the interest obligation associated with these notes so that the interest payable on the notes effectively became fixed at a rate of 3.32%.