Retirement Plans (Reconciliation of Benefit Obligations And Plan or Trust Assets And Resulting Funded Status Of Defined Benefit Retirement Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Change in Benefit Obligation
Benefit obligation at beginning of year	$ (18,107)	$ (15,720)
Service cost	(115)	(111)	$ (180)
Interest cost	(491)	(600)	(532)
Amendments	37	(3)
Actuarial gain (loss)	(1,780)	(2,446)
Settlement/curtailment/other	(65)	(33)
Benefits paid	829	806
Benefit obligation at end of year	(19,692)	(18,107)	(15,720)
Change in Plan/Trust Assets
Fair value of assets at beginning of year	13,177	11,532
Actual return on plan assets	1,843	2,206
Employer contributions	480	185
Settlement/curtailment/other	58	39
Benefits paid	(807)	(785)
Fair value of assets at end of year	14,751	13,177	11,532
Funded status at end of year	(4,941)	(4,930)
Other  Post-retirement Benefits
Change in Benefit Obligation
Benefit obligation at beginning of year	(1,027)	(935)
Service cost	(10)	(8)	(10)
Interest cost	(27)	(35)	(33)
Amendments	2	(8)
Actuarial gain (loss)	(60)	(101)
Settlement/curtailment/other	(4)	(4)
Benefits paid	64	64
Benefit obligation at end of year	(1,062)	(1,027)	(935)
Change in Plan/Trust Assets
Fair value of assets at beginning of year	644	570
Actual return on plan assets	102	117
Employer contributions	0	2
Settlement/curtailment/other	0	0
Benefits paid	(41)	(45)
Fair value of assets at end of year	705	644	$ 570
Funded status at end of year	$ (357)	$ (383)