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Long-Term Debt - Narrative (Details)
Jun. 21, 2018
USD ($)
Sep. 30, 2018
USD ($)
Revolving Credit Facility    
Debt Instrument [Line Items]    
Debt instrument, unused borrowing capacity, amount   $ 1,420,000,000
Covenant through June 30, 2019 | Amended and Restated Competitive Advance and Revolving Credit Agreement    
Debt Instrument [Line Items]    
Covenant requirement, maximum total leverage ratio 5.0  
Covenant from September 30, 2019 to June 30, 2020 | Amended and Restated Competitive Advance and Revolving Credit Agreement    
Debt Instrument [Line Items]    
Covenant requirement, maximum total leverage ratio 4.75  
Covenant from September 30, 2020 and thereafter | Amended and Restated Competitive Advance and Revolving Credit Agreement    
Debt Instrument [Line Items]    
Covenant requirement, maximum total leverage ratio 4.50  
Line of Credit | Amended and Restated Competitive Advance and Revolving Credit Agreement    
Debt Instrument [Line Items]    
Maximum borrowing capacity $ 1,510,000,000.00