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Debt (Details)
12 Months Ended
Dec. 01, 2016
USD ($)
Dec. 31, 2017
USD ($)
line_of_credit
financial_institution
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Short-term Debt [Line Items]        
Short-term debt outstanding   $ 565,745,000 $ 512,970,000  
Proceeds from lines of credit   75,000,000 0 $ 0
Commercial Paper [Member]        
Short-term Debt [Line Items]        
Short-term debt outstanding   $ 491,800,000 $ 513,000,000  
Weighted average interest rate   1.60% 0.90%  
Average daily borrowings   $ 482,200,000 $ 422,900,000  
Weighted average interest rates on average daily borrowings   1.20% 0.60%  
Credit facility, maximum borrowing capacity   $ 1,000,000,000.0    
Credit Facility [Member]        
Short-term Debt [Line Items]        
Credit facility, maximum borrowing capacity   1,000,000,000.0    
Incremental increases in principal   250,000,000.0    
Amounts outstanding under credit facility   0 $ 0  
Proceeds from lines of credit   0 0  
Credit Facility [Member] | SCB LLC [Member]        
Short-term Debt [Line Items]        
Credit facility, maximum borrowing capacity   $ 175,000,000    
Number of uncommitted lines of credit | line_of_credit   3    
Number of financial institutions offering lines of credit | financial_institution   3    
Number of financial institutions with stated limits | line_of_credit   2    
Uncommitted Lines of Credit [Member]        
Short-term Debt [Line Items]        
Short-term debt outstanding   $ 0 0  
Average daily borrowings   $ 4,500,000 $ 4,400,000  
Weighted average interest rate   1.40% 1.10%  
Credit Facility [Member] | Revolving Credit Facility [Member]        
Short-term Debt [Line Items]        
Credit facility, maximum borrowing capacity $ 200,000,000      
Amounts outstanding under credit facility   $ 75,000,000 $ 0  
Interest Rate   2.40%    
Term of debt instrument 364 days      
Average daily borrowings   $ 21,400,000 $ 7,300,000  
Weighted average interest rate   2.00% 1.60%