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Benefit Plans Components of Benefit Plans (Details)
$ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2020
USD ($)
Mar. 31, 2019
USD ($)
Mar. 31, 2020
USD ($)
plans
Mar. 31, 2019
USD ($)
Retirement Plan Disclosure        
Number of unfunded post-retirement health care plans | plans     2  
Pension Plan        
Retirement Plan Disclosure        
Service cost $ 15 $ 10 $ 27 $ 22
Interest cost 104 126 208 249
Defined Benefit Plan, Expected Return (Loss) on Plan Assets (122) 120 (244) 239
Amortization of prior service cost (24) (24) (48) (49)
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) 109 (86) 218 (168)
Net periodic benefit cost as acutarially determined 82 78 161 151
Amount capitalized due to actions of regulator (6) 2 (8) (1)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) 76 76 153 152
Other Postretirement Benefit Plan        
Retirement Plan Disclosure        
Service cost 4 3 8 6
Interest cost 4 5 8 9
Defined Benefit Plan, Expected Return (Loss) on Plan Assets 0   0 0
Amortization of prior service cost (6) (6) (12) (12)
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) 3 (1) 5 (2)
Net periodic benefit cost as acutarially determined 5 3 9 5
Amount capitalized due to actions of regulator 0   0 0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) $ 5 $ 3 $ 9 $ 5