Benefit Plans (Amounts Recorded in Accumulated Other Comprehensive Losses) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net loss	$ (3,274)	$ (2,999)
Prior service (cost) credit	9	40
Deferred income taxes	845	767
Amounts recorded in accumulated other comprehensive losses	(2,420)	(2,192)
Pension [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net loss	(2,689)	(2,565)
Prior service (cost) credit	(27)	(27)
Deferred income taxes	702	670
Amounts recorded in accumulated other comprehensive losses	(2,014)	(1,922)
Postretirement Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net loss	(541)	(389)
Prior service (cost) credit	41	72
Deferred income taxes	132	86
Amounts recorded in accumulated other comprehensive losses	(368)	(231)
Postemployment Retirement Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net loss	(44)	(45)
Prior service (cost) credit	(5)	(5)
Deferred income taxes	11	11
Amounts recorded in accumulated other comprehensive losses	$ (38)	$ (39)