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Debt (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Mar. 31, 2016
Line of Credit Facility [Line Items]      
Line of Credit Facility, Maximum Borrowing Capacity $ 1,025.0 $ 650.0  
Letters of Credit Outstanding, Amount 18.2 18.2  
Borrowing capacity remaining $ 66.3    
Unamortized Debt Issuance Expense   18.1  
Line of Credit [Member]      
Line of Credit Facility [Line Items]      
Line of Credit Facility, Maximum Borrowing Capacity     $ 2,000.0
Covenant terms, minimum leverage ratio through September 2013 350.00%    
Covenant Terms Minimum Leverage Ratio Through Specified Date Following a Material Acquisition 400.00%    
Long-Term Senior Notes [Member]      
Line of Credit Facility [Line Items]      
Debt Instrument, Face Amount $ 275.0 $ 275.0  
Debt Instrument, Interest Rate, Stated Percentage 4.40% 4.40%  
Unamortized Debt Issuance Expense $ 2.3 $ 2.7  
Line of Credit [Member]      
Line of Credit Facility [Line Items]      
Line of Credit Facility, Maximum Borrowing Capacity $ 2,750.0    
Pricing on the Facility based on market rates LIBOR plus 0.95% to 2.05%    
Description of variable rate basis LIBOR    
Basis spread on variable rate 1.05%    
Average outstanding borrowings $ 981.6 $ 335.1  
Line of credit, interest rate during period 1.50% 1.10%  
Covenant terms, minimum cash interest coverage ratio 300.00%    
Unamortized Debt Issuance Expense $ 19.6 $ 15.4