Retirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 13	$ (47)	$ (109)
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	357	405	387
Interest cost	883	802	842
Expected return on assets	(1,225)	(1,252)	(1,196)
Prior service cost (credit)	0	2	1
Actuarial loss	414	629	607
Settlement and other charges	17	3	3
Contractual Termination Benefits	1	0	0
Net periodic benefit cost	447	589	644
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	93	112	106
Interest cost	188	175	182
Expected return on assets	(153)	(173)	(162)
Prior service cost (credit)	(179)	(186)	(188)
Actuarial loss	45	66	61
Settlement and other charges	1	1	0
Contractual Termination Benefits	0	0	0
Net periodic benefit cost	$ (5)	$ (5)	$ (1)