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Credit Facility (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Line of Credit Facility [Line Items]          
Line of Credit Facility, Maximum Borrowing Capacity $ 1,125.0   $ 1,125.0   $ 1,025.0
Letters of Credit Outstanding, Amount 11.9   11.9   $ 18.2
Line of Credit [Member]          
Line of Credit Facility [Line Items]          
Line of Credit Facility, Maximum Borrowing Capacity 2,750.0   2,750.0    
Line of Credit Facility, Average Outstanding Amount $ 1,302.8 $ 941.5 $ 1,210.6 $ 747.9  
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)     1.25%    
Line of Credit Facility, Interest Rate During Period 2.00% 1.50% 1.90% 1.50%