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Debt and Other Obligations Debt and Other Obligations - Credit Facility Agreements (Details)
Sep. 30, 2018
USD ($)
Credit_facilities
Sep. 30, 2017
USD ($)
Credit Facility Agreements    
Current borrowing capacity $ 150,000,000  
Line of Credit    
Credit Facility Agreements    
Current borrowing capacity 150,000,000  
Credit facility agreements borrowings outstanding 0  
Revolving Credit Facilities    
Credit Facility Agreements    
Current borrowing capacity 2,700,000,000  
Credit facility agreements borrowings outstanding $ 0  
Number of revolving credit facilities | Credit_facilities 4  
Revolving Credit Facility 4 $ 150,000,000  
Revolving credit facility 3 500,000,000  
Revolving credit facility 1 1,000,000,000  
Revolving Credit Facility 2 1,000,000,000  
Long-term Line of Credit, Borrowings 4 0  
Long-term Line of Credit, Borrowings 3 0  
Long-term Line of Credit, Borrowings 1 0  
Long-term Line of Credit, Borrowings 2 0  
Line of Credit Facility, Remaining Borrowing Capacity 4 112,000,000  
Line of Credit Facility, Remaining Borrowing Capacity 3 0  
Line of Credit Facility, Remaining Borrowing Capacity 1 617,000,000  
Line of Credit Facility, Remaining Borrowing Capacity 2 1,000,000,000  
Line of Credit Facility, Remaining Borrowing Capacity 1,729,000,000  
Letter of Credit    
Credit Facility Agreements    
Amount of letters of credit outstanding 900,000,000 $ 1,200,000,000
Letters of Credit Outstanding, Amount 4 38,000,000  
Letters of Credit Outstanding, Amount 3 500,000,000  
Letters of Credit Outstanding, Amount 1 383,000,000  
Letter of Credit Outstanding, Amount 2 $ 0