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Benefit Plans Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended 9 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2019
Mar. 31, 2018
Jun. 30, 2017
Sep. 30, 2019
Other Pension Plan          
Retirement Plan Disclosure          
Defined benefit plan contributions   $ 150     $ 25
Pension Benefits          
Retirement Plan Disclosure          
Service cost $ 10 22 $ 13 $ 27  
Interest cost 126 249 119 237  
Expected return on plan assets 120 239 (119) (239)  
Amortization of prior service cost (24) (49) (24) (49)  
Recognized net actuarial loss (86) (168) 101 204  
Total net periodic benefit cost as actuarially determined 78 151 90 180  
Amount capitalized due to actions of regulator 2 (1) (13) (27)  
Total net periodic benefit cost 76 152 77 153  
Other Post-retirement Benefits          
Retirement Plan Disclosure          
Defined benefit plan contributions   150      
Service cost 3 6 3 7  
Interest cost 5 9 4 9  
Expected return on plan assets 0 0 0 0  
Amortization of prior service cost (6) (12) (5) (11)  
Recognized net actuarial loss (1) 2 2 4  
Total net periodic benefit cost as actuarially determined 3 5 4 9  
Amount capitalized due to actions of regulator 0 0 0 0  
Total net periodic benefit cost $ 3 $ 5 $ 4 $ 9