Retirement Benefits 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, 2018	Dec. 31, 2017
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service (credit) cost	$ (29)	$ (24)
Actuarial loss	7,558	7,556
Total	7,529	7,532
Benefits included in accumulated other comprehensive income	3,899	3,896
Benefits included in regulatory assets	3,630	3,636
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service (credit) cost	(337)	(522)
Actuarial loss	531	829
Total	194	307
Benefits included in accumulated other comprehensive income	70	125
Benefits included in regulatory assets	$ 124	$ 182