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Benefit Plans Components of Benefit Plans (Details)
$ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2021
USD ($)
Mar. 31, 2020
USD ($)
Mar. 31, 2021
USD ($)
plans
Mar. 31, 2020
USD ($)
Retirement Plan Disclosure        
Number of unfunded post-retirement health care plans | plans     2  
Pension Plan        
Retirement Plan Disclosure        
Service cost $ 14 $ 15 $ 28 $ 27
Interest cost 93 104 184 208
Defined Benefit Plan, Expected Return (Loss) on Plan Assets (123) 122 (246) 244
Amortization of prior service cost (25) (24) (49) (48)
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) 115 (109) 227 (218)
Net periodic benefit cost as acutarially determined 74 82 144 161
Amount capitalized due to actions of regulator (2) 6 (9) (8)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) 76 76 153 153
Other Postretirement Benefit Plan        
Retirement Plan Disclosure        
Service cost 4 4 9 8
Interest cost 4 4 8 8
Defined Benefit Plan, Expected Return (Loss) on Plan Assets 0   0 0
Amortization of prior service cost (4) (6) (9) (12)
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) 2 (3) 5 (5)
Net periodic benefit cost as acutarially determined 6 5 13 9
Amount capitalized due to actions of regulator 0   0 0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) $ 6 $ 5 $ 13 $ 9