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Benefit Plans Projected Benefit Obligations and Accumulated Benefit Obligations in Exess of Plan Assets (Details) - Pension Benefits - USD ($)
$ in Millions
Sep. 30, 2020
Sep. 30, 2019
Defined Benefit Plan Disclosure    
Projected benefit obligation   $ 13,312
Accumulated benefit obligation $ 13,613 13,246
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Plan Assets 7,959 7,980
Fair value of net plan assets $ 7,980 $ 7,980