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Retirement Plans (Details)
$ in Millions
 12 Months Ended
Sep. 30, 2019
USD ($)
Person
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Retirement Plans [Line Items]      
Defined benefit plan, pension plans with accumulated benefit obligations in excess of plan assets, aggregate projected benefit obligation $ 5,450 $ 5,166  
Defined benefit plan, pension plans with accumulated benefit obligations in excess of plan assets, aggregate accumulated benefit obligation 5,388 5,072  
Defined benefit plan, pension plans with accumulated benefit obligations in excess of plan assets, aggregate fair value of plan assets 4,484 4,525  
Total employer and employee contributions to defined benefit plans 50    
Voluntary contributions made by the company to defined benefit plans 0    
Estimated future employer contributions to defined benefit plans in next fiscal year $ 50    
Modification in salaried plans to limit company's cost of future annual retiree medical benefits 150.00%    
Projected prescription drug subsidy receipts per year over the next ten years $ 1    
Matching contributions charged to expense for defined contribution savings plans $ 198 205 $ 190
Number of multiemployer benefit plans in which the Company participates | Person 285    
Employer contributions to multiemployer pension plans $ 69 68 67
Postretirement Benefits      
Retirement Plans [Line Items]      
Defined Benefit Plan, Benefit Obligation 174 $ 196 $ 214
Total employer and employee contributions to defined benefit plans 3    
Estimated future employer contributions to defined benefit plans in next fiscal year $ 4