Retirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Amortization of:
Net periodic benefit cost	$ 53	$ 13	$ (47)
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	387	357	405
Interest cost	757	883	802
Expected return on assets	(1,270)	(1,225)	(1,252)
Amortization of:
Prior service cost (credit)	4	0	2
Actuarial loss	512	414	629
Curtailment benefits	0	0	0
Settlement and other charges	14	17	3
Contractual termination benefits	0	1	0
Net periodic benefit cost	404	447	589
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	90	93	112
Interest cost	154	188	175
Expected return on assets	(163)	(153)	(173)
Amortization of:
Prior service cost (credit)	(124)	(179)	(186)
Actuarial loss	49	45	66
Curtailment benefits	(1)	0	0
Settlement and other charges	1	1	1
Contractual termination benefits	0	0	0
Net periodic benefit cost	$ 6	$ (5)	$ (5)