Retirement Benefits - Gross Accumulated Other Comprehensive Loss and Regulatory Assets (Liabilities) not Recognized as Components of Periodic Benefit Cost (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit)	$ 35	$ 39
Actuarial loss	8,077	7,662
Total	8,112	7,701
Benefits included in accumulated other comprehensive income	4,339	4,068
Benefits included in regulatory assets	3,773	3,633
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit)	(145)	(158)
Actuarial loss	538	565
Total	393	407
Benefits included in accumulated other comprehensive income	183	177
Benefits included in regulatory assets	$ 210	$ 230