Retirement Benefits and Trusteed Assets (Pension Plan - Pension Cost Inclusions) (Details) - Pension plan - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		$ 99	$ 84	$ 99
Interest cost		186	219	202
Expected return on plan assets		(334)	(325)	(329)
Amortization of:
Net actuarial loss		171	131	176
Prior service cost		1	1	0
Settlements	$ 22	25	2	0
Net pension cost/other postretirement cost (credit)		$ 148	$ 112	$ 148