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Retirement Plans - Changes in the Fair Value of Pension Assets (Details) - Pension plan - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Change in Plan Assets:    
Fair value of plan assets at beginning of year $ 470 $ 449
Fair value of plan assets at end of year 486 470
Significant Unobservable Inputs (Level 3)    
Change in Plan Assets:    
Fair value of plan assets at beginning of year 6  
Fair value of plan assets at end of year 6 6
Real estate | Significant Unobservable Inputs (Level 3)    
Change in Plan Assets:    
Fair value of plan assets at beginning of year 6 6
Redemptions 0 0
Fair value of plan assets at end of year $ 6 $ 6