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Retirement Plans (Details) - USD ($)
 3 Months Ended
Jun. 30, 2019
Mar. 31, 2020
Mar. 31, 2019
Net Periodic Pension And OPEB Cost [Abstract]      
Changes in fair value   5 years  
Defined Benefit Plan, Accumulated Benefit Obligation, Increase (Decrease) for Plan Amendment $ (92,000,000)    
Other comprehensive income defined benefit plans 69,000,000 $ (22,000,000) $ (10,000,000)
Deferred Tax Assets, Gross $ 23,000,000    
Future net peroidic OPEB cost amortization duration 8 years    
Defined Benefit Plans [Member]      
Net Periodic Pension And OPEB Cost [Abstract]      
Service cost   27,000,000 22,000,000
Interest cost   34,000,000 40,000,000
Expected return on plan assets   (70,000,000) (68,000,000)
Amortization of actuarial loss   28,000,000 16,000,000
Net periodic cost   19,000,000 10,000,000
Voluntary cash contributions to qualified pension plan   0  
OPEB [Member]      
Net Periodic Pension And OPEB Cost [Abstract]      
Interest cost   1,000,000 3,000,000
Amortization of prior service credit   (3,000,000)  
Amortization of actuarial loss   $ 2,000,000 1,000,000
Net periodic cost     $ 4,000,000