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Benefit Plans Components of Benefit Plans (Details)
$ in Millions
 3 Months Ended
Dec. 31, 2020
USD ($)
plans
Dec. 31, 2019
USD ($)
Retirement Plan Disclosure    
Number of unfunded post-retirement health care plans | plans 2  
Pension Plan    
Retirement Plan Disclosure    
Service cost $ 14 $ 12
Interest cost 91 104
Defined Benefit Plan, Expected Return (Loss) on Plan Assets (123) 122
Amortization of prior service cost (24) (24)
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) 112 (109)
Net periodic benefit cost as acutarially determined 70 79
Amount capitalized due to actions of regulator (7) 2
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) 77 77
Other Postretirement Benefit Plan    
Retirement Plan Disclosure    
Service cost 5 4
Interest cost 4 4
Defined Benefit Plan, Expected Return (Loss) on Plan Assets 0 0
Amortization of prior service cost (5) (6)
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) 3 (2)
Net periodic benefit cost as acutarially determined 7 4
Amount capitalized due to actions of regulator 0 0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) $ 7 $ 4