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Benefit Plans Contributions (Details) - USD ($)
 12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Defined Benefit Plan Disclosure      
Other postretirement benefit contributions   $ 47,000,000 $ 44,000,000
Supplemental Employee Retirement Plans, Defined Benefit      
Defined Benefit Plan Disclosure      
Employer contributions   6,000,000 7,000,000
Other Post-retirement Benefits      
Defined Benefit Plan Disclosure      
Employer contributions [1]   41,000,000 43,000,000
Pension Benefits      
Defined Benefit Plan Disclosure      
Employer contributions   281,000,000 282,000,000
Other Pension Plans, Defined Benefit      
Defined Benefit Plan Disclosure      
Employer contributions   275,000,000 275,000,000
Minimum | Other Pension Plans, Defined Benefit      
Defined Benefit Plan Disclosure      
Employer contributions   $ 209,000,000 $ 215,000,000
Scenario, Forecast      
Defined Benefit Plan Disclosure      
Other postretirement benefit contributions $ 35,000,000    
Scenario, Forecast | Supplemental Employee Retirement Plans, Defined Benefit      
Defined Benefit Plan Disclosure      
Employer contributions 5,000,000    
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 federal reinsurance payments and provider discounts and rebates.