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Retirement Plans Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]      
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), after Reclassification Adjustment, before Tax $ 6    
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), after Reclassification Adjustment, before Tax   $ 1  
Postretirement Benefits      
Components of Net Periodic Benefit Cost:      
Service cost 1 2 $ 2
Interest cost 6 7 6
Expected return on plan assets (9) (10) (10)
Net actuarial (gain) loss (17) (5) 5
Curtailment gain 0 0 0
Settlement (gain) loss 0 0 0
Net periodic benefit cost (credit) $ (15) $ (4) $ 7
Expense Assumptions:      
Discount rate 3.80% 3.70% 3.30%
Expected return on plan assets 5.65% 5.65% 5.60%
Discontinued Operations | Postretirement Benefits      
Components of Net Periodic Benefit Cost:      
Net periodic benefit cost (credit) $ 0 $ 2 $ (2)
Continuing Operations [Member] | Postretirement Benefits      
Components of Net Periodic Benefit Cost:      
Net periodic benefit cost (credit) $ (15) $ (2) $ 5