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Benefit Plans Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Sep. 30, 2019
Dec. 31, 2018
Dec. 31, 2017
Sep. 30, 2019
Other Pension Plan        
Retirement Plan Disclosure        
Defined benefit plan contributions $ 75 $ 13   $ 25
Pension Benefits        
Retirement Plan Disclosure        
Service cost   12 $ 14  
Interest cost   123 118  
Expected return on plan assets   (119) (120)  
Amortization of prior service cost   (25) (25)  
Recognized net actuarial loss   82 103  
Total net periodic benefit cost as actuarially determined   73 90  
Amount capitalized due to actions of regulator   3 (14)  
Total net periodic benefit cost   76 76  
Other Post-retirement Benefits        
Retirement Plan Disclosure        
Service cost   3 4  
Interest cost   4 5  
Expected return on plan assets   0 0  
Amortization of prior service cost   (6) (6)  
Recognized net actuarial loss   1 2  
Total net periodic benefit cost as actuarially determined   2 5  
Amount capitalized due to actions of regulator   0 0  
Total net periodic benefit cost   $ 2 $ 5  
Scenario, Forecast | Other Pension Plan        
Retirement Plan Disclosure        
Defined benefit plan contributions       225
Scenario, Forecast | Other Post-retirement Benefits        
Retirement Plan Disclosure        
Defined benefit plan contributions       $ 300