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Benefit Plans Other Postemployment Benefits (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Other Post-Employment Benefits      
Discount rate 1.68% 3.05% 2.33%
Period expense $ 59 $ 6  
Postemployment benefits liability 419 339  
Accounts Payable and Accrued Liabilities      
Other Post-Employment Benefits      
Postemployment Benefits Liability, Current 36 39  
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent      
Other Post-Employment Benefits      
Postemployment benefits liability, noncurrent $ 383 $ 360