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Benefit Plans Contributions (Details) - USD ($)
 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Defined Benefit Plan Disclosure      
Employer contributions     $ 500,000,000
Other postretirement benefit contributions   $ 25,000,000 30,000,000
Contribution related to TVARS case   15,000,000 20,000,000
Expected Payment for Postretirement Benefits   29,000,000  
Supplemental Employee Retirement Plans, Defined Benefit      
Defined Benefit Plan Disclosure      
Employer contributions     5,000,000
Defined Benefit Plan, Expected Payments Related to SERP   6,000,000  
Defined Benefit Plan, Related to SERP   4,000,000  
Other Post-retirement Benefits      
Defined Benefit Plan Disclosure      
Employer contributions [1]   36,000,000 36,000,000
Pension Benefits      
Defined Benefit Plan Disclosure      
Employer contributions   304,000,000 805,000,000
Other Pension Plans, Defined Benefit      
Defined Benefit Plan Disclosure      
Employer contributions   300,000,000 $ 800,000,000
Minimum | Other Pension Plans, Defined Benefit      
Defined Benefit Plan Disclosure      
Employer contributions   $ 300,000,000  
Scenario, Forecast | Other Pension Plans, Defined Benefit      
Defined Benefit Plan Disclosure      
Employer contributions $ 300,000,000    
[1] Other Post-Retirement Benefits Employer contributions are reduced by provider discounts and rebates.