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Benefit Plans Components of Net Periodic Benefit Cost (Details) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Other Pension Plan              
Retirement Plan Disclosure              
Defined benefit plan contributions       $ 225,000,000     $ 800,000,000
Pension Benefits              
Retirement Plan Disclosure              
Service cost   $ 13,000,000 $ 15,000,000 40,000,000 $ 45,000,000    
Interest cost   118,000,000 116,000,000 355,000,000 348,000,000    
Expected return on plan assets   (119,000,000) (114,000,000) (358,000,000) (342,000,000)    
Amortization of prior service cost   (25,000,000) (25,000,000) (74,000,000) (74,000,000)    
Recognized net actuarial loss   103,000,000 118,000,000 307,000,000 354,000,000    
Total net periodic benefit cost as actuarially determined   90,000,000 110,000,000 270,000,000 331,000,000    
Amount capitalized due to actions of regulator   (13,000,000) (34,000,000) (40,000,000) (102,000,000)    
Total net periodic benefit cost   77,000,000 76,000,000 230,000,000 229,000,000    
Other Post-retirement Benefits              
Retirement Plan Disclosure              
Defined benefit plan contributions       17,000,000 22,000,000    
Service cost   4,000,000 5,000,000 11,000,000 14,000,000    
Interest cost   5,000,000 4,000,000 14,000,000 15,000,000    
Expected return on plan assets   0 0 0 0    
Amortization of prior service cost   (5,000,000) (5,000,000) (16,000,000) (16,000,000)    
Recognized net actuarial loss   1,000,000 3,000,000 5,000,000 10,000,000    
Total net periodic benefit cost as actuarially determined   5,000,000 7,000,000 14,000,000 23,000,000    
Amount capitalized due to actions of regulator   0 0 0 0    
Total net periodic benefit cost   $ 5,000,000 $ 7,000,000 $ 14,000,000 $ 23,000,000    
Scenario, Forecast | Other Pension Plan              
Retirement Plan Disclosure              
Defined benefit plan contributions $ 75,000,000         $ 300,000,000