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Credit Facility (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Line of Credit Facility [Line Items]      
Long-term Line of Credit, Noncurrent, Net of Debt Issuance Costs $ 310.8   $ (15.3)
Line of Credit Facility, Maximum Borrowing Capacity 850.0   850.0
Letters of Credit Outstanding, Amount 8.6   9.0
Line of Credit [Member]      
Line of Credit Facility [Line Items]      
Long-term Line of Credit, Noncurrent, Net of Debt Issuance Costs 310.8   $ (15.3)
Line of Credit Facility, Maximum Borrowing Capacity 2,750.0    
Line of Credit Facility, Average Outstanding Amount $ 202.3 $ 1,117.5  
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 (in hundredths) 1.05%    
Line of Credit Facility, Interest Rate During Period 2.60% 1.70%