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Debt (Credit Facilities) (Details)
1 Months Ended
Jun. 30, 2018
USD ($)
extension
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Line of Credit Facility [Line Items]      
Balance availability under Credit Facilities   $ 1,694,100,000 $ 1,639,100,000
Borrowings outstanding from Credit Facilities   159,000,000.0 130,000,000.0
Letters of credit outstanding under our Credit Facilities   379,600,000 462,700,000
Long-term debt   8,445,200,000  
Credit facilities: May 2021      
Line of Credit Facility [Line Items]      
Credit facility maximum borrowing capacity $ 1,000,000,000.0    
Credit facilities: June 2019      
Line of Credit Facility [Line Items]      
Credit facility maximum borrowing capacity 1,250,000,000    
Uncommitted revolver, amended borrowing capacity      
Line of Credit Facility [Line Items]      
Credit facility maximum borrowing capacity     135,000,000.0
Credit facilities: Uncommitted facility      
Line of Credit Facility [Line Items]      
Long-term debt   $ 33,400,000 $ 0
Line of Credit | Revolving Credit Facility      
Line of Credit Facility [Line Items]      
Credit facility maximum borrowing capacity $ 2,250,000,000    
Number of available extensions | extension 2    
Extension period 1 year    
Accordion feature, increase limit (up to) $ 1,000,000,000.0