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, 2019	Dec. 31, 2018
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service (credit) cost	$ 39	$ (29)
Actuarial loss	7,662	7,558
Total	7,701	7,529
Benefits included in accumulated other comprehensive income	4,068	3,899
Benefits included in regulatory assets	3,633	3,630
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service (credit) cost	(158)	(337)
Actuarial loss	565	531
Total	407	194
Benefits included in accumulated other comprehensive income	177	70
Benefits included in regulatory assets	$ 230	$ 124