Debt and Other Obligations Debt and Other Obligations - Credit Facility Agreements (Details)	Sep. 30, 2025 USD ($) Credit_facilities	Sep. 30, 2024 USD ($)
Credit Facility Agreements
Amount of letters of credit outstanding	$ 6,000,000
Borrowings under U.S. Treasury credit facility	0
Revolving Credit Facilities
Credit Facility Agreements
Current borrowing capacity	2,650,000,000
Credit facility agreements borrowings outstanding	$ 0
Number of revolving credit facilities | Credit_facilities	4
Revolving Credit Facility 4	$ 150,000,000
Revolving credit facility 1	500,000,000
Long-term Line of Credit, Borrowings 4	0
Long-term Line of Credit, Borrowings 1	0
Line of Credit Facility, Remaining Borrowing Capacity 4	112,000,000
Line of Credit Facility, Remaining Borrowing Capacity 1	285,000,000
Long-term Line of Credit, Borrowings 3	0
Revolving credit facility 3	1,000,000,000
Line of Credit Facility, Remaining Borrowing Capacity	2,152,000,000
Line of Credit Facility, Remaining Borrowing Capacity 2	890,000,000
Long-term Line of Credit, Borrowings 2	0
Revolving Credit Facility 2	1,000,000,000
Line of Credit Facility, Remaining Borrowing Capacity 3 true	865,000,000
Letter of Credit
Credit Facility Agreements
Amount of letters of credit outstanding	498,000,000	$ 566,000,000
Letters of Credit Outstanding, Amount 3	135,000,000
Letters of Credit Outstanding, Amount 4	38,000,000
Letters of Credit Outstanding, Amount 1	215,000,000
Letter of Credit Outstanding, Amount 2	110,000,000
Letters of Credit Outstanding, Total	498,000,000
Line of Credit
Credit Facility Agreements
Current borrowing capacity	$ 150,000,000