Retirement Plans (Net Periodic Pension And OPEB Cost) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension [Member]
Net Periodic Pension And OPEB Cost [Abstract]
Service cost	$ 91	$ 80	$ 105
Interest cost	137	160	145
Expected return on plan assets	(282)	(273)	(272)
Amortization of prior service cost/(credit)
Amortization of actuarial loss	104	67	93
Net periodic pension/OPEB cost	50	34	71
OPEB [Member]
Net Periodic Pension And OPEB Cost [Abstract]
Service cost	1	1	2
Interest cost	5	9	10
Expected return on plan assets
Amortization of prior service cost/(credit)	(14)	(7)	1
Amortization of actuarial loss	7	7	10
Net periodic pension/OPEB cost	$ (1)	$ 10	$ 23