Retirement Benefits - Calculation of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Amortization of:
Net periodic benefit cost	$ (15)	$ 2	$ (38)	$ (8)
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	97	89	290	267
Interest cost	190	221	569	663
Expected return on assets	(317)	(306)	(953)	(918)
Amortization of:
Prior service cost (benefit)	1	0	3	0
Actuarial loss	128	104	384	310
Settlement charges	8	7	14	7
Contractual Termination Benefits	0	1	0	1
Net periodic benefit cost	107	116	307	330
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	22	23	67	70
Interest cost	37	47	114	141
Expected return on assets	(41)	(38)	(122)	(115)
Amortization of:
Prior service cost (benefit)	(30)	(45)	(92)	(134)
Actuarial loss	12	11	36	34
Settlement charges	0	0	0	0
Contractual Termination Benefits	0	0	0	0
Net periodic benefit cost	$ 0	$ (2)	$ 3	$ (4)