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Debt and Credit Agreements Long-Term Debt (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Long-term debt, gross $ 1,287.6 $ 1,604.6
Long-term debt, current maturities 2.0 323.9
Long-term debt, excluding current maturities 1,285.6 1,280.7
Deb instrument, unamortized debt issuance costs 13.0 12.3
Long-term debt, fair value 1,342.6  
2.25% Notes    
Debt Instrument [Line Items]    
Debt instrument, face amount 300.0  
Long-term debt, gross 0.0 299.4
Debt instrument, unamortized discount 0.0  
Deb instrument, unamortized debt issuance costs 0.0  
Long-term debt, fair value [1] $ 0.0 301.4
Debt instrument, maturity date Nov. 15, 2017  
Interest rate, stated percentage 2.25%  
Debt instrument, interest rate, effective percentage 2.30%  
Total cash paid to redeem 2.25% Note $ 303.4  
Accrued and unpaid interest 3.4  
4.00% Notes    
Debt Instrument [Line Items]    
Debt instrument, face amount 250.0  
Long-term debt, gross 247.6 247.0
Debt instrument, unamortized discount 1.3  
Deb instrument, unamortized debt issuance costs 1.1  
Long-term debt, fair value [1] $ 259.0 258.4
Debt instrument, maturity date Mar. 15, 2022  
Interest rate, stated percentage 4.00%  
Debt instrument, interest rate, effective percentage 4.13%  
3.75% Notes    
Debt Instrument [Line Items]    
Debt instrument, face amount $ 500.0  
Long-term debt, gross 497.1 496.6
Debt instrument, unamortized discount 0.8  
Deb instrument, unamortized debt issuance costs 2.1  
Long-term debt, fair value [1] $ 513.2 503.3
Debt instrument, maturity date Feb. 15, 2023  
Interest rate, stated percentage 3.75%  
Debt instrument, interest rate, effective percentage 4.32%  
4.20% Notes    
Debt Instrument [Line Items]    
Debt instrument, face amount $ 500.0  
Long-term debt, gross 496.7 496.2
Debt instrument, unamortized discount 0.7  
Deb instrument, unamortized debt issuance costs 2.6  
Long-term debt, fair value [1] $ 524.2 511.6
Debt instrument, maturity date Apr. 15, 2024  
Interest rate, stated percentage 4.20%  
Debt instrument, interest rate, effective percentage 4.24%  
Notes Payable, Other Payables [Member]    
Debt Instrument [Line Items]    
Long-term debt, gross $ 46.2 65.4
Long-term debt, fair value [1] $ 46.2 $ 65.4
[1] See Note 10 for information on the fair value measurement of our long-term debt.