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Credit Facility (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Line of Credit Facility [Line Items]          
Long-term Line of Credit, Noncurrent, Net of Debt Issuance Costs $ 397.2   $ 397.2   $ (15.3)
Line of Credit Facility, Maximum Borrowing Capacity 62.5   62.5   850.0
Letters of Credit Outstanding, Amount 8.6   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 397.2   397.2   $ (15.3)
Line of Credit Facility, Maximum Borrowing Capacity 2,750.0   2,750.0    
Line of Credit Facility, Average Outstanding Amount $ 523.0 $ 1,302.8 $ 363.5 $ 1,210.6  
Pricing on the Facility based on market rates     LIBOR plus 0.875% to 1.35%    
Description of Variable Rate Basis     LIBOR    
Basis spread on variable rate (in hundredths)     1.00%    
Line of Credit Facility, Interest Rate During Period 2.80% 2.00% 2.70% 1.90%