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Long-Term Debt and Commercial Paper (Narrative) (Details) - USD ($)
$ in Millions
1 Months Ended 3 Months Ended 12 Months Ended
Apr. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]      
Letters of credit, amount outstanding   $ 47.9  
Commercial paper, maximum aggregate amount outstanding permitted   1,000.0  
Commercial paper, amount outstanding   $ 270.0 $ 330.0
Commercial Paper [Member]      
Debt Instrument [Line Items]      
Weighted-average annual interest rate   2.26% 1.97%
Maximum [Member] | Commercial Paper [Member]      
Debt Instrument [Line Items]      
Maturity period of debt   397 days  
Weighted Average [Member] | Commercial Paper [Member]      
Debt Instrument [Line Items]      
Maturity period of debt   14 days 24 days
3.35% Senior Notes due 2021 [Member] | Senior Notes [Member]      
Debt Instrument [Line Items]      
Senior notes   $ 300.0 $ 300.0
Debt instrument, maturity date   Jan. 15, 2021  
Percentage interest on debt instrument   3.35%  
3.5% Senior Notes Due 2024 [Member] | Senior Notes [Member]      
Debt Instrument [Line Items]      
Senior notes   $ 450.0 450.0
Debt instrument, maturity date   Nov. 15, 2024  
Percentage interest on debt instrument   3.50%  
4.5% Senior Notes due 2025 [Member] | Senior Notes [Member]      
Debt Instrument [Line Items]      
Senior notes   $ 450.0 450.0
Debt instrument, maturity date   Oct. 01, 2025  
Percentage interest on debt instrument   4.50%  
3.8% Senior Notes Due 2027 [Member] | Senior Notes [Member]      
Debt Instrument [Line Items]      
Senior notes   $ 300.0 300.0
Debt instrument, maturity date   Nov. 15, 2027  
Percentage interest on debt instrument   3.80%  
6.75% Senior Notes Due 2018 [Member] | Senior Notes [Member]      
Debt Instrument [Line Items]      
Senior notes   $ 400.0 400.0
Debt instrument, maturity date   Apr. 15, 2018  
Percentage interest on debt instrument   6.75%  
6.75% Senior Notes Due 2018 [Member] | Senior Notes [Member] | Subsequent Event [Member]      
Debt Instrument [Line Items]      
Repayment of 6.75% Senior Debt $ 400.0    
5.5% Senior Notes Due 2020 [Member] | Senior Notes [Member]      
Debt Instrument [Line Items]      
Senior notes   $ 350.0 350.0
Debt instrument, maturity date   Feb. 01, 2020  
Percentage interest on debt instrument   5.50%  
Revolving Credit Facility Due 2022 [Member]      
Debt Instrument [Line Items]      
Maximum borrowing capacity under revolving credit facility   $ 1,800.0  
Additional borrowing capacity under accordion feature of revolving credit facility   500.0  
Revolving credit facility, amount outstanding   0.0 0.0
Revolving credit facilities letter of credit sublimit   200.0  
Additional borrowing capacity under the revolving credit facility   1,800.0  
Borrowing capacity limited under the maximum consolidated leverage ratio   $ 1,400.0  
Leverage ratio, minimum threshold, current credit spread   2.0  
Leverage ratio, maximum threshold, current credit spread   3.25  
Leverage ratio, minimum threshold, increase in credit spread   3.25  
Impact on credit spread from increase in leverage ratio   0.125%  
Revolving Credit Facility Due 2022 [Member] | Minimum [Member]      
Debt Instrument [Line Items]      
Commitment fee on undrawn amounts (percent)   0.15%  
Revolving Credit Facility Due 2022 [Member] | Maximum [Member]      
Debt Instrument [Line Items]      
Commitment fee on undrawn amounts (percent)   0.25%  
Revolving Credit Facility Due 2022 [Member] | Line of Credit [Member]      
Debt Instrument [Line Items]      
Debt instrument, maturity date   Oct. 19, 2022  
Revolving Credit Facility Due 2022 [Member] | Line of Credit [Member] | London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]      
Debt Instrument [Line Items]      
Basis spread on variable interest rates (percent)   1.25%  
Revolving Credit Facility Due 2022 [Member] | Line of Credit [Member] | London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]      
Debt Instrument [Line Items]      
Basis spread on variable interest rates (percent)   1.625%  
Revolving Credit Facility Due 2022 [Member] | Line of Credit [Member] | Base Rate [Member] | Minimum [Member]      
Debt Instrument [Line Items]      
Basis spread on variable interest rates (percent)   0.25%  
Revolving Credit Facility Due 2022 [Member] | Line of Credit [Member] | Base Rate [Member] | Maximum [Member]      
Debt Instrument [Line Items]      
Basis spread on variable interest rates (percent)   0.625%  
Capital Leases and Other Debt [Member]      
Debt Instrument [Line Items]      
Capital leases and other debt   $ 138.1 $ 139.4