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Fair Value Measurements - Carrying Amounts and Estimated Fair Values of Long-term Debt (Details) - USD ($)
$ in Millions
Mar. 31, 2024
Sep. 30, 2023
Mar. 31, 2023
Carrying amount      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Long-term debt $ 6,826 $ 6,647 $ 6,805
Estimated fair value      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Long-term debt $ 6,647 $ 6,238 $ 6,480