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Business Transformation Initiatives (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Mar. 31, 2021
Mar. 31, 2020
Restructuring and Related Activities [Abstract]        
Costs principally comprising consulting, advisory and employee-related costs $ 17 $ 19 $ 32 $ 36