Retirement Benefits - Summary of Changes in Benefit Obligations and Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Net benefit obligation at beginning of year	$ 20,692	$ 22,337
Service cost	357	405	$ 387
Interest cost	883	802	842
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant	0	0
Actuarial (gain) loss(a)	2,322	(1,561)
Plan amendments	68	(4)
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment	(3)	0
Settlements	(35)	(48)
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits	1	0
Defined Benefit Plan, Benefit Obligation, Benefits Paid	(1,417)	(1,239)
Net benefit obligation at end of year	22,868	20,692	22,337
Fair value of net plan assets at beginning of year	16,678	18,573
Actual return on plan assets	3,008	(945)
Employer contributions	356	337
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant	0	0
Defined Benefit Plan, Plan Assets, Benefits Paid	(1,417)	(1,239)
Defined Benefit Plan, Plan Assets, Amount	18,590	16,678	18,573
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Net benefit obligation at beginning of year	4,369	4,856
Service cost	93	112	106
Interest cost	188	175	182
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant	44	45
Actuarial (gain) loss(a)	250	(540)
Plan amendments	0	0
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment	0	0
Settlements	(4)	(4)
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits	0	0
Defined Benefit Plan, Benefit Obligation, Benefits Paid	(282)	(275)
Net benefit obligation at end of year	4,658	4,369	4,856
Fair value of net plan assets at beginning of year	2,408	2,732
Actual return on plan assets	324	(136)
Employer contributions	51	46
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant	44	45
Defined Benefit Plan, Plan Assets, Benefits Paid	(282)	(275)
Defined Benefit Plan, Plan Assets, Amount	$ 2,541	$ 2,408	$ 2,732