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Fair Value Measurements - (Details) - USD ($)
$ in Millions
Mar. 31, 2021
Sep. 30, 2020
Mar. 31, 2020
Carrying amount      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Long-term debt $ 6,046 $ 6,081 $ 5,879
Estimated fair value      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Long-term debt $ 6,362 $ 6,504 $ 5,736