Retirement Benefits - Summary of Changes in Benefit Obligations and Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Net benefit obligation at beginning of year	$ 22,868	$ 20,692
Service cost	387	357	$ 405
Interest cost	757	883	802
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant	0	0
Actuarial loss(a)	2,217	2,322
Plan amendments	0	68
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment	0	(3)
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement	(45)	(35)
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits	0	1
Defined Benefit Plan, Benefit Obligation, Benefits Paid	(1,290)	(1,417)
Net benefit obligation at end of year	24,894	22,868	20,692
Fair value of net plan assets at beginning of year	18,590	16,678
Actual return on plan assets	2,547	3,008
Employer contributions	542	356
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant	0	0
Defined Benefit Plan, Plan Assets, Benefits Paid	(1,290)	(1,417)
Defined Benefit Plan, Plan Assets, Payment for Settlement	(45)	(35)
Defined Benefit Plan, Plan Assets, Amount	20,344	18,590	16,678
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Net benefit obligation at beginning of year	4,658	4,369
Service cost	90	93	112
Interest cost	154	188	175
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant	49	44
Actuarial loss(a)	49	250
Plan amendments	(111)	0
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment	0	0
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement	(5)	(4)
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits	0	0
Defined Benefit Plan, Benefit Obligation, Benefits Paid	(280)	(282)
Net benefit obligation at end of year	4,604	4,658	4,369
Fair value of net plan assets at beginning of year	2,541	2,408
Actual return on plan assets	190	324
Employer contributions	59	51
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant	49	44
Defined Benefit Plan, Plan Assets, Benefits Paid	(280)	(282)
Defined Benefit Plan, Plan Assets, Payment for Settlement	(5)	(4)
Defined Benefit Plan, Plan Assets, Amount	$ 2,554	$ 2,541	$ 2,408