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Debt and Credit Agreements Credit Agreements (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Line of Credit Facility [Line Items]    
Cash debt pooling arrangements, amount netted $ 2,702.2 $ 2,274.9
Margin for base rate advances 0.125%  
Margin for Eurocurrency rate advances 1.125%  
Annual rate for fronting fees accrued on letters of credit 0.25%  
Annual facility fee 0.125%  
Leverage ratio maximum 4.25x  
Debt Instrument, Fee Amount $ 2.0  
364 Day [Member]    
Line of Credit Facility [Line Items]    
Margin for base rate advances 0.25%  
Margin for Eurocurrency rate advances 1.25%  
Annual facility fee 0.25%  
Line of credit facility, current borrowing capacity $ 500.0  
Line of Credit Facility, Fair Value of Amount Outstanding 0.0  
Committed credit facility [Member]    
Line of Credit Facility [Line Items]    
Line of credit facility, maximum borrowing capacity 1,500.0  
Line of credit facility, available capacity increase amount 250.0  
Limits on letters of credit 50.0  
Line of credit facility, amount outstanding 0.0  
Letters of credit, amount outstanding 9.0 8.4
Line of credit facility, remaining borrowing capacity 1,491.0 1,491.6
Uncommitted credit facility    
Line of Credit Facility [Line Items]    
Line of credit facility, amount outstanding 48.0 52.4
Line of Credit Facility, Average Outstanding Amount 86.5 88.0
Line of credit facility, current borrowing capacity $ 857.6 $ 1,056.0
Line of credit facility, weighted average interest rate, over time 3.60% 5.20%